The FBI on Thursday searched the home of a relative of San Bernardino shooter Syed Farook, the agency said.
Officials would not discuss the nature of the search at the Corona, Calif., apartment or what authorities are looking for.
The warrant was specific to the residence, not any individuals, a law enforcement source said. The search ended at 10 a.m., and no arrests have been made.
Farook and his wife, Tashfeen Malik, opened fire on a conference room full of county workers on Dec. 2, killing 14 people in what authorities believe was a terrorism-inspired attack.
Farook and Malik were killed in a shootout with police hours after the massacre at the Inland Regional Resource Center.
While authorities would not characterize the search Thursday, the FBI has been trying to piece together where the couple went after the shooting and what motivated them to carry out the attack.
On Tuesday that investigation sparked a larger fight over information security when a judge ordered Apple give investigators technical assistance to access to encrypted data on Farook's iPhone.
Apple is fighting the order, accusing the federal government of an "overreach" that could put the privacy of millions of other cell phone users at risk.
Apple CEO Tim Cook called the ruling a "dangerous precedent." Cook said the government is asking Apple to "build a backdoor to the iPhone" — something he described as "too dangerous to create."
"Once created, the technique could be used over and over again, on any number of devices," Cook said.
Farook and Malik opened fire on a meeting of the San Bernardino County Department of Public Health, which was hosting a holiday luncheon and training session at the social services agency.
Farook worked at the health department, and prosecutors argued evidence in Farook's iCloud account indicates that he was in communication with victims whom he and his wife later shot.
Phone records show Farook communicated with Malik using his iPhone, prosecutors said.