The FBI is investigating the fatal shooting of 14 people in California by a married couple as an "act of terrorism," officials said Friday, noting the wife was believed to have pledged allegiance to a leader of the militant Islamic State group.

While the FBI said it lacked evidence the couple belonged to a larger organization of extremists, the Los Angeles Times cited a federal law enforcement source in reporting that the husband, Syed Rizwan Farook, 28, had contact with at least two militant groups overseas, including the al-Qaida-affiliated Nusra Front in Syria.

Both the U.S.-born husband and his spouse, Tashfeen Malik, 29, a native of Pakistan who lived in Saudi Arabia for more than 20 years, died in a shootout with police hours after Wednesday's attack on a holiday party at the Inland Regional Center social services agency in San Bernardino, about 60 miles (100 km) east of Los Angeles.