Federal agents in Riverside conducted a search early on Saturday at what neighbors say was the house of a childhood friend of Syed Rizwan Farook, who with his wife killed 14 people in a mass shooting in California on Wednesday.
The searched house was located next door to the home in which Farook lived with his family for more than a decade.
Federal Bureau of Investigation spokeswoman Laura Eimiller confirmed that a federal search warrant had been executed in Riverside overnight, but declined to give the address or comment on the reason for the search.
A source close to the investigation said the search took place at the home of Enrique Marquez, and that investigators were looking into the relationship between Marquez and Farook. Marquez has not been charged with any crime.
Four neighbors interviewed on Saturday said that Marquez was a good friend of Farook.
Farook, 28, and his wife, Tashfeen Malik, 29, most recently lived in Redlands, a community about 30 miles (48 km) northeast of Riverside. The couple died in a shootout with police hours after their attack on a party for civil servants in nearby San Bernardino.
U.S. authorities are investigating the shootings as an act of terrorism, and on Saturday Islamic State said the couple were followers of the militant group.