The couple accused of killing 14 people at a holiday party in California gave no indication of their plans ahead of the deadliest mass shooting in the United States in three years, a lawyer for the deceased shooters' family said on Friday.

Syed Rizwan Farook, 28, and his wife, Tashfeen Malik, 27, were killed in a shootout with police hours after the Wednesday massacre at the Inland Regional Center social services agency in San Bernardino, about 60 miles (100 km) east of Los Angeles.

"They never saw any warning signs or any real characteristics that would let them know that any of this was going to take place," family attorney David Chelsey said on CNN. "There's so little there to understand or explain what happened or why it happened."

The couple left behind a 6-month old daughter. Farook's brother-in-law, Farhan Khan, told NBC News he had begun legal proceedings to adopt the girl and was "very upset and angry" at Farook.

"You left your 6-month-old daughter," Khan said. "In this life some people cannot have kids. God gave you a gift of a daughter. And you left that kid behind ... What did you achieve?"

Twenty-one people were wounded in the attack, the worst gun violence in the nation since the December 2012 shooting at Sandy Hook Elementary School in Newtown, Connecticut, in which 27 people died.

The motive for the attack is unclear. Officials including President Barack Obama and San Bernardino Police Chief Jarrod Burguan have said it may have been motivated by extremist ideology but that no concrete information has emerged suggesting ties to Islamic State or other militant groups.

Farook, a U.S. citizen born in Illinois, was the son of Pakistani immigrants, according to Hussam Ayloush, head of the Los Angeles area chapter of the Muslim advocacy group Council on American-Islamic Relations.

Malik was a Pakistani native living in Saudi Arabia when they married, Ayloush said.

Investigators are reviewing the couples' computers, cellphones and other electronic devices to see if they had browsed jihadist websites or had contact with militant groups, according to officials in Washington familiar with the investigation, which is being led by the Federal Bureau of Investigation.

Police said the couple had two assault-style rifles, two semi-automatic handguns and 1,600 rounds of ammunition in their vehicle when they were killed. A search of their home turned up another 4,500 rounds and 12 pipe bombs.