— -- Law enforcement officers returned to the home of a former neighbor of San Bernardino shooting suspect Syed Farook today, and left with a cardboard box but wouldn't answer any questions.
Enrique Marquez Jr., Farook's former neighbor, is believed to have purchased two of the guns used in the shooting Wednesday that left 14 people dead and 21 wounded, law enforcement sources told ABC News. The FBI has said that Marquez is not considered a suspect in the shooting.
The three officers arrived at Marquez' Riverside, California, home at 1:30 p.m. and were let in by a man inside the home. They were inside for roughly 15 minutes before they left with a box.
Police previously said that two handguns used in the attack were purchased by Farook and his wife-turned-accomplice, Tashfeen Malik, but the other two guns -– modified "assault-style" weapons -- had been purchased by an unidentified third party. The law enforcement official and a source close to the investigation told ABC News Saturday the third party is suspected to be Marquez.
Investigators are still trying to get a clearer picture of what drove Farook and Malik.
The rampage Wednesday is now a federal terrorism investigation, with officials saying that Farook and Malik appear to have been inspired by ISIS, but FBI Director James Comey said there is no indication the two are part of an organized larger group, cell or network.
Still, federal authorities told ABC News they are investigating whether Farook may have left behind social media connections with one of the most active and significant ISIS recruiters, an American from Minneapolis now an FBI fugitive living overseas by the name of Mohamed Hasan who uses the on-line handle Miski.
Earlier this year Miski was in direct contact with the two terrorists who attacked a Mohammed cartoon contest in Garland, Texas. He tweeted to his American followers then that others should take up arms.
Authorities say they increasingly believe that 29-year-old Tashfeen Malik, not her husband, was the mastermind and instigator of the attack. U.S. investigators in Pakistan are now giving her background close scrutiny.
Sources told ABC News that Malik, using an alias on social media, allegedly pledged her allegiance to Abu Bakr al-Baghdadi, the leader of ISIS.
Malik, who is from Pakistan but had lived in Saudi Arabia, came to the U.S. last summer on a "fiancée" visa and later obtained a Green Card, U.S. officials have said. Malik married Farook, an American of Pakistani descent, in August 2014 and the couple had a baby daughter six months ago.
Suspects Farook and Malik were killed by police in a shootout after their deadly attack. Sources told ABC News that in addition to having plenty of ammunition for another attack, the couple also made sure to cover their tracks.
Police detailed the extent of the arsenal of a dozen pipe bomb-style devices and more than 4,500 rounds of ammunition that were found at the couple's Redlands, California, home, as well as the 1,600 rounds they had in the rental car they were driving during their face-off with police.
ABC News' Pierre Thomas, Josh Margolin, James Gordon Meek, Tom Liddy and Sally Hawkins contributed to this report.
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