SAN BERNARDINO, Calif. — 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 might 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 nabbed four local men in November of that year for plotting to kill Americans in Afghanistan. The four men were convicted and sentenced to long prison terms.

Agents are investigating whether those four 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.

RIFLES BOUGHT FOR 2012 ATTACK?

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 hear, aware that just because Marquez is telling them something doesn’t make it true, a senior U.S. law enforcement source 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 they have not detailed what they would be. There is no sign that Marquez has retained an attorney. Officials say there’s 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.

MULTIPLE TARGETS HYPOTHESIZED

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, along with 12 pipe bombs. And there was an unsuccessful attempt to modify one of the weapons to make it shoot automatically, according to officials.

Also unknown is where the killers were during a four-hour period between the massacre and their final stand on San Bernardino Ave. They were seen driving by their rented apartment in Redlands, 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.

Authorities on Sunday called off their search of a lake in San Bernardino where they think one of the shooters discarded a computer hard drive. Although material has been pulled out by police divers, the FBI has not said whether anything significant has been found.

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