Police want to speak to a friend of San Bernardino shooter Syed Rizwan Farook who purchased the two semi-automatic rifles used in the massacre, NBC News has learned.
Authorities say the man bought the guns legally in 2011 and 2012, each time shortly after Farook bought a handgun.
All four guns were used in Wednesday's rampage by Farook and his wife, Tashfeen Mailk, that left 14 people dead at a gathering of county employees.
It is not known whether the purchases were coordinated or how Farook came into possession of the rifles bought by his friend.
The two .223-caliber assault rifles were a DPMS model and a Smith & Wesson MMP 15 model, while the two 9 mm semi-automatic handguns were manufactured by llama and Smith & Wesson.
As NBC News reported earlier Friday, one of the rifles appeared to have been modified in an attempt to allow it to fire fully automatic.
Government field tests showed that the modification to fully automatic mode was performed but failed, a senior law enforcement source told NBC News.
A second rifle was subsequently changed to have a large-capacity detachable magazine which would be illegal under California law, the source said.
The suspects used and carried what appeared to be multiple, 30-round rifle magazines, some of which were taped together, the source said.