"He would say stuff like: 'There’s so much going on. There’s so many sleeper cells, so many people just waiting. When it happens, it’s going to be big. Watch.'"

"We took it as a joke. When you look at the kid and talk to him, no one would take him seriously about that.”

Said a regular at Morgan’s Tavern, about Enrique Marquez, who worked there.

Federal investigators believe that more than any other witness, Mr. Marquez, a convert to Islam, has “held the keys” to understanding what motivated [Syed] Farook and his wife, Tashfeen Malik....

On behalf of Mr. Farook, Mr. Marquez bought the two assault rifles used in the attack, the authorities say. He told investigators he had done so because Mr. Farook believed he could not pass a background check. Mr. Marquez has also described in detail how he and Mr. Farook had been planning another terrorist attack together in 2012....

Last year, Mr. Marquez married the Russian sister of Raheel Farook’s wife. He later told a friend and people at the bar that it was a sham marriage for immigration purposes. He told bar patrons he had been paid $5,000 or $10,000 to marry Ms. Gigliotti’s sister, Mariya Chernykh. He announced the arrangement one day when he came into the bar — which the F.B.I. visited this week — and offered to buy everyone drinks, Mr. Rodriguez, the bar patron, said.... He told Mr. Rodriguez that he had posted photographs of himself and his wife for the sake of appearances, but that Ms. Chernykh lived at her own apartment and would not so much as kiss him....

Shortly after the shooting, Mr. Marquez’s friends noticed a cryptic and poorly written post on his Facebook page: “I’m. Very sorry sguys. It was a pleasure.”