The San Bernardino killers' network of interest.

When Enrique Marquez didn’t show up on Friday, Dec. 4, for his shift as a doorman at Morgan’s Tavern, a pirate-themed neighborhood bar in nearby Riverside that’s popular with the punk-rock crowd, his co-workers became concerned. Then they saw a strange, garbled note that he had posted on his Facebook page after midnight that Thursday:

“I’m. Very sorry sguys. It was a pleasure.”

The co-workers feared that he may have become suicidal. In fact, he had checked himself into a mental-health facility sometime in the immediate aftermath of the Dec. 2 shooting rampage here in which a husband and wife killed 14 people using assault rifles that Marquez had purchased several years earlier.

Marquez, who was quickly tracked down by the FBI, has been cooperating with the extensive investigation of the massacre carried out by Marquez’s former neighbor Syed Rizwan Farook and Farook’s Pakistani wife, Tashfeen Malik.

According to senior U.S. law enforcement officials, Marquez has told the FBI that he and Farook had discussed mounting some sort of attack in 2012, but then he got spooked after a terrorism investigation based in Riverside resulted in the arrest of four local men in November of that year for plotting to kill Americans in Afghanistan. The men were convicted and sentenced to long prison terms.

A sign is seen in the front yard after law enforcement officials raided the Riverside home of Enrique Marquez, who lived next door to an old address of shooter Syed Rizwan Farook. (Jabin Botsford/The Washington Post)

Agents are investigating whether those men or any of their associates — or the FBI’s confidential informant in that case — had contact with Farook or Marquez. Lawyers involved in the case said they were unaware of a direct connection.

It is not clear whether Farook and Marquez chose a specific target or time to carry out the attack they had discussed, the officials said. The FBI is investigating whether the rifles that Marquez bought were intended for use in that 2012 attack that was called off, the officials said.

If they were — or if Marquez knew at the time when he transferred the rifles to Farook that they were going to be used for a violent act — he could be charged with a federal felony, according to law enforcement officials. Authorities do not believe that Marquez had any direct knowledge of the later plot by Farook and Malik.

As they question Marquez, they are “running to ground” everything they are hearing, aware that just because Marquez is telling them something doesn’t make it true, a senior U.S. law enforcement official said.

Marquez’s whereabouts are unknown during this period of interrogation. He appears to be in legal limbo. Authorities expect to file charges against him but have not detailed what they would be. There is no sign Marquez has retained an attorney. Officials say there is no urgency to arrest him as long as he is cooperating and they are verifying what he is saying.

Eleven days after the attack, the number of arrests remains zero. The scope of the conspiracy remains unknown. The global investigation involves scores of police officers and federal agents from the FBI; the Bureau of Alcohol, Tobacco, Firearms and Explosives; and the U.S. Marshals Service. They are methodically trying to piece together the history of Farook, Malik, Marquez and any associates who may have known anything about the couple and their terrorist plot.

Investigators think that the Inland Regional Center, where Farook and Malik attacked Farook’s co-workers at a holiday gathering, was not their main target. The attack lasted just four minutes, in which the shooters fired off 65 to 75 rounds and then fled in their rented SUV.

The shooters potentially could have killed many more in the building if that was their only target. They had more than 1,600 rounds in their vehicle when they were killed in a gun battle with police. They had thousands more rounds of ammunition at their house, as well as material that could be used to make bombs. And there was an unsuccessful attempt to modify one weapon to make it shoot automatically, according to law enforcement officials.

Family and friends remembered the 14 victims of the Dec. 2 shooting attack in San Bernardino, Calif., as "hard-working, family-loving, gentle people." All but one worked for the San Bernardino County Department of Public Health with attacker Syed Rizwan Farook. (Jenny Starrs/The Washington Post)

Also unknown is the location of the killers during a four-hour period between the massacre and their final stand on San Bernardino Avenue. They were seen driving by their apartment in Redlands, a small city about 10 miles to the east, and then drove away, pursued by police. Farook and Malik drove back to San Bernardino — directly toward the site of their rampage.

The FBI announced Sunday that material retrieved by divers from a lake near the massacre site is being analyzed as possible evidence. The FBI had been tipped that the shooters had possibly discarded something — authorities think it was a computer hard drive or other electronics — on the day of the attack. There are no plans to search the lake further, the bureau said.

Marquez, 24, is literally the boy next door in this unfolding narrative: He lived for many years with his family in a one-story beige house directly next to Farook. Both attended La Sierra High School in Riverside, but several years apart. Neighbors say the two young men would spend hours at a time dismantling and repairing cars on the driveway of Farook’s house.

Marquez converted to Islam about 4 1/ years ago and for a time worshiped at a mosque in Corona.

Yousuf Bhaghani, president of the board of the mosque, said Marquez attended about four years ago but then stopped coming. “Some people made the comment that he was goofy,” he said.

Sometime after that, Marquez and Farook began talking about an attack of some kind, and that’s when Marquez bought the assault rifles, which were legally purchased, authorities have said. Marquez never filled out paperwork transferring ownership to Farook as required by law, and it is unclear when he gave Farook the weapons.

“Farook used him. He got him to buy the guns because he already had in mind to use them,” theorized Rosie Aguirre, a neighbor who lives across the street.

There is another, somewhat mysterious, connection between Marquez and Farook, and it involves two Russian women. Farook’s brother, a Navy veteran named Syed Raheel Farook, is married to a Russian woman, Tatiana Farook, who had worked for some time as a hairdresser in Corona. Her sister, Mariya Chernykh, married Marquez last year — although a co-worker of Marquez said it was an arranged marriage and a strained relationship.

Marquez’s family has declined multiple requests for interviews, but his mother, Armida Chacon, briefly talked Thursday to reporters, becoming distraught as she described her son as “a good person.” The home still shows damage from a post-midnight police raid on the weekend after the shooting when investigators broke into the garage and seized property thought to be related to the case. A piece of broken wallboard has been propped up by the mailbox as a sign with a polite request for people stay off the property.

Marquez’s Facebook page shows nothing that would fit any stereotype of a terrorist. The profile photo is of Marquez with an exaggerated, goofy smile.

Marquez worked as a Walmart security guard and moonlighted at Morgan’s Tavern, which is tucked in the corner of a small retail strip that is off the beaten track in Riverside.

Jerry Morgan, owner of the tavern, said Marquez worked at the door to check IDs on band nights.

“People ask me, ‘What did he do?’ ” Morgan said. “I say, ‘You stand at the door and ask him for his ID.’ There’s his job description. Vomit on the carpet? He was the guy that cleaned it up. Cigarette butts in the parking lot? He was that guy. Quiet guy.”

Another co-worker, who spoke of the condition of anonymity because of the sensitive nature of the investigation, described him as meek and, echoing something her boss had said, “unable to punch his way out of a wet paper bag.”

Goldman reported from Washington. Rob Kuznia in San Bernardino and Julie Tate in Washington contributed to this report.