As the investigation into the Twin Towers bombing enters its second week, federal officials say their only certainty is that Mohammed A. Salameh did not act alone. But the breadth of the alleged conspiracy, its motive and possible links to state-sponsored terrorism all remain a muddle. Salameh, charged by the government last Thursday with aiding and abetting the blast that killed five and injured more than 1,000, was a man who authorities say kept dangerous company. The FBI is looking hard at his links to a radical Muslim sect headed by Sheik Omar Abdel-Rahman, a blind Egyptian cleric whose fiery rhetoric may have led to violence in the past. Salameh, who maintains his innocence, worshiped at the Jersey City storefront mosque where Rahman occasionally preaches. The mosque also attracted another figure well known to counterterrorist investigators. El Sayyid Nosair stood trial in New York for the 1990 assassination of Rabbi Meir Kahane, founder of the Jewish Defense League. He was acquitted, but is serving a long prison term on weapons charges related to the case. Among his visitors in jail, say federal law-enforcement sources, has been Salameh.

Officials believe that these are not casual associations. “It’s more than a coincidence that the two men prayed in the same mosque,” said a knowledgeable State Department official. “This was not St. Patrick’s Cathedral. It was a very small mosque.” Investigators may be overstating the case. In fact, the mosque, one of two in Jersey City’s thriving Islamic community, attracts hundreds of worshipers each week. And at the weekend the government had no evidence connecting Salameh and Rahman.

President Clinton applauded the apparent early break in the case and assured Americans that “massive” legal resources were at work on the bombing. But NEWSWEEK has learned that the administration is far more shaken than it has publicly let on. Intelligence officials have privately warned the administration that even if the attack proves to be the work of freelancers acting without foreign sponsorship, it may attract more seasoned international operatives who perceive a new American vulnerability to domestic terrorism. “The problem is that now a lot of bad folks overseas know that even though this guy screwed up afterwards, he still pulled it off,” said a senior administration official. At the weekend the administration was assembling a package of enhanced counterterrorism measures it may announce as early as this week. Among the moves under consideration are increased funding for surveillance hardware and improved technology for cross-checking visa applications to screen travelers for terrorist connections.

While Washington worried about the global consequences, investigators on the ground in New York waded deeper into the case. On Friday evening, federal agents opened a storage locker that they say was rented in Salameh’s name and contained chemical components for urea nitrate, a substance commonly used in explosives. Police are also studying items they seized Thursday after searching the Jersey City apartment where Salameh reportedly lived. A federal complaint says agents found wiring, electromagnetic devices and other bomb-related tools.

Immediately after the blast, experts assumed it could take weeks, perhaps months, to find out what really happened. Forensic evidence remained buried under dangerous, unstable debris. But by some violent caprice, a calling card was left at the blast’s perimeter-the partial truck chassis. Experts knew that the pattern of damage on the piece’s surface linked it to the heart of the explosion.

The government’s good fortune was exceeded only by Salameh’s astonishing naivete. On Feb. 23, three days before the bombing, according to the federal complaint, Salameh and another unidentified man rented a yellow Ford Econoline van in Salameh’s name from a Ryder truck-rental agency on Kennedy Boulevard in Jersey City. Officials say Salameh even demanded a vehicle without a broken side mirror, telling a rental agent he didn’t want to be stopped by police. Two hours after the explosion, he returned to the Ryder office to report that the van had been stolen in a supermarket parking lot the previous evening. He asked for a refund of his $400 cash deposit but Pat Galasso, the Ryder agent, told him he would have to file a police report first. Later that evening police told Salameh they couldn’t complete the report until he produced the truck’s license number. Last Monday, Salameh returned to Galasso without a report and angrily renewed his demand for the money. Galasso again refused; Salameh said he would be back on Wednesday.

Salameh didn’t show up on Wednesday, but the FBI did. Had he been a faithful reader of New York Newsday, he might have been able, at least, to delay his arrest. On Thursday morning, the tabloid reported that agents were looking into the yellow van from Jersey City. Investigators, who had planned to keep Salameh under a surveillance that they hoped would lead to others, moved to nab him quickly. The FBI asked Galasso to telephone Salameh and offer him his money. At 10:15, Salameh walked into the office. “Hey, Mohammed, what’s happening?” Galasso said, introducing him to an undercover agent. “This is a Ryder rep to help you with your claim.” Minutes later Salameh was in handcuffs. In his pocket, authorities said, was a copy of the rental agreement with traces of bomb residue. “I hate to say he masterminded this,” says one FBI source. “I would use the word ’no-mind’.”

In the aftermath of Salameh’s dramatic arrest, some officials complained that Newsday had forced authorities to move too quickly. The FBI says some individuals wanted for questioning have disappeared. “There is growing concern that the arrest was entirely premature,” says one federal law-enforcement official. Newsday says that both the FBI and the New York Police were aware of the story the previous evening and registered no objections.

At the weekend authorities investigating the case had one other man in custody-Ibraham Elgabrowny, a cousin of Sayyid Nosair’s whose Brooklyn address appears on Salameh’s New York driver’s license, according to federal investigators. Officials say that Elgabrowny, charged with obstruction of justice for allegedly punching two FBI agents during a search of his apartment, plunged his hands into a urine-filled toilet before investigators could test for traces of explosives. Agents say the search also suggested that Elgabrowny might have been planning to help spring Nosair from prison. Agents found a pistol and a phony Nicaraguan passport in Nosair’s name. Elgabrowny was being held without bail, but a senior Justice Department official said he is not a suspect in the bombing. “He’s nothing,” the official said.

Salameh himself remains a mysterious figure. A Palestinian born near the Israeli-held West Bank town of Nablus, he came to the United States on a Jordanian passport in 1987. Neighbors in his Kensington Avenue apartment in Jersey City remember him as a slight, quietly devout man with a trim beard and dark, close-cropped curly hair who often wore blue jeans and sweat shirts. “He fit in,” said Joe Vigliotti, a computer operator for the Bank of America. “He didn’t seem to have this big thing he had to do on his mind. . . . There was no weird vibe.” He shared a first-floor apartment with Josie Hadas, a fiftyish woman who residents say cooked and did laundry for a fluid group of young men who kept late hours. Hadas has not been charged in the case.

Some officials strongly suspect that Salameh is merely a foot soldier in a larger conspiracy. “Salameh was a corporal,” said one U.S. counterterrorist official. “He didn’t do it alone. He needed help gathering the explosives undetected. He needed help building the bomb. Someone had to conduct the intelligence to know where to place the bomb … Everything points to an operation designed to have a maximum effect with a very measured and very contemplated end result.”

Scores of unanswered questions remain buried under hundreds of tons of rubble at the WTC garage. Investigators hope to start extracting answers this week when they get better access to the site. Experts place the epicenter of the blast at the foot of the garage’s B2 ramp, suggesting that the van may not have been in a parking space when it blew up. Officials postulate that the van may have hit a speed bump, triggering a premature explosion. Dogs at the site have picked up what might be the scent of decaying bodies. Some investigators believe they could be the remains of one or more of Salameh’s alleged collaborators.

The government got lucky last week. The telltale chassis, combined with the hapless Salameh, created an early advantage. But it will require more than good fortune to piece together the rest of the story-at the scene, in Jersey City and, perhaps, in the capitals of the Middle East.

PHOTO: Fiery rhetoric from a Jersey City mosque: Rahman (MIKE SEGAR–REUTERS)

PHOTO: An early trophy: Investigators quickly linked a piece of truck chassis found in the debris to the van they believe carried the bomb (TIM CLARY–AFP)

Bill Clinton called it an investigation that should “reassure the American people.” A look at how round-the-clockwork and good luck led to the first big break in the case:

At 12:18 p.m., an explosion rocks the World Trade Center. At 1:35 a caller claiming to be from the Serbian Liberation Front tells NYPD the blast “is no accident.” Of more than 50 such calls, none is deemed credible-all came after the bombing. Firefighters rescue victims trapped in the crater before forensic work begins late Friday night.

Combing through debris at the edge of the crater, investigators late Sunday night find a three-foot, soot-covered piece of metal on level B2, part of one of 200 cars and trucks destroyed by the bomb. Later, forensic evidence places the vehicle at the seat of the blast. With a pieced-together Vehicle Identification Number, FBI traces the yellow Ford van to a truck-leasing company.

FBI begins stakeout of the Jersey City, N.J., Ryder dealership where the yellow van was rented. The manager says the young man who last rented the van returned twice to report it stolen and to try to collect his deposit. At 6 p.m. editors from New York Newsday Inform FBI they will publish Information about the Jersey City connection.

FBI resumes Jersey City stakeout at 8:10 a.m. Prompted by a phone call from the manager, Mohammed Salameh, 25, arrives at 10:15 to collect a $200 refund of the deposit on the yellow Ford van he had rented and reported stolen. Eight FBI agents arrest Salameh as he leaves the office. Searching his apartment, bomb-sniffing dogs respond to the scent in a closet. Information collected during the search leads agents the next day to a self-storage locker they say was rented by Salameh. Inside they would find chemicals used to make bombs. At 7:42 p.m., Salameh pleads not guilty at his Manhattan arraignment.

A second man, Ibraham Elgabrowny, 42, is arrested in an afternoon raid at a Brooklyn address police say was once used by both Salameh and convicted gunman El Sayyid Nosair, who was acquitted in the 1990 murder of Rabbi Meir Kahane. Elgabrowny is accused of punching FBI agents.