The husband-and-wife attackers who shot to death 14 people at a California holiday party fired as many as 75 shots in the horrific rampage and left behind a remote-controlled toy car rigged with three pipe bombs that failed to detonate, police said Thursday.
San Bernardino Police Chief Jarrod Burguan said the couple, a Chicago-born local government health inspector and his Pakistani wife, escaped in a rented sport utility vehicle Wednesday before police killed them in a violent gunbattle three kilometers away on a street in nearby Redlands.
Burguan said the man, Syed Rizwan Farook, 28, and his wife, Tashfeen Malik, 27, fired 75 rifle shots at police, with the police returning 380 rounds at them. When the carnage ended, Burguan said police found 1,600 rounds of ammunition on their bodies or in their car.
WATCH: San Bernardino police chief discusses shooting rampage
In addition, Burguan said investigators discovered 12 pipe bombs in the couple's apartment, bomb-making materials and another 4,500 or more rounds of ammunition.
Federal Bureau of Investigation official David Bowdich said Farook had traveled overseas and returned to the United States with Malik in July 2014, with the woman entering the U.S. on a Pakistani passport. Bowdich said that the couple later married and that she was in the United States on a visa.
The investigators said Thursday that it was too soon to say what the couple's motive was for launching the massacre. Neither was known to have a criminal record, and they were not on any watch list.
President Barack Obama said terrorism or a workplace dispute could have motivated the couple to carry out the attack, which left 21 others injured.
Speaking at the White House, Obama said, "It is possible that this was terrorist-related, but we don't know. It's also possible that this was workplace-related."
Obama added, "I can assure the American people we're going to get to the bottom of this," with investigators looking for clues as to the motives. The FBI's Bowdich said investigators were scouring the couple's digital devices for evidence.
WATCH: President Obama's statement on California shooting
Obama ordered American flags on government buildings, embassies and military installations flown at half-staff until Monday to honor the shooting victims.
He said the assault at a local government social services center for developmentally disabled people should spur lawmakers in Washington "to take basic steps to make it harder, not impossible, to get weapons."
Obama said that "right now it's just too easy" for people to buy guns in the United States, where gun ownership is an embedded right in the Constitution, and that American society is going to have to search itself to figure out a way to curb gun violence. Burguan said that all four guns the couple were carrying were legally purchased and registered.
Obama offered his comments on the shooting after receiving briefings from the country's top law enforcement officials, Attorney General Loretta Lynch and the FBI's director, James Comey.
Burguan said the couple were dressed in "assault-style clothing" with ammunition attached and heavily armed with assault weapons and semiautomatic handguns when they stormed into the Inland Regional Center, located 90 kilometers from Los Angeles.
The police chief said Farook had been at the party earlier but left, possibly after a dispute. He said Farook then returned to the facility, which provides services for people with such disabilities as autism, cerebral palsy and epilepsy.
'On a mission'
"They came prepared to do what they did," Burguan said, "as if they were on a mission. Based on how they were equipped, there had to be some degree of planning that went into this. I don't think they just ran home and put on these tactical clothes, grabbed guns and came back on a spur-of-the-moment thing."
"There was obviously a mission here," the FBI's Bowdich said. "We know that. We do not know why. We don't know if this was the intended target or if there was something that triggered him to do this immediately."
The couple had dropped off their 6-month-old daughter with relatives before the attack, saying they had a doctor's appointment.
San Bernardino Police Lieutenant Mike Madden was the first officer to arrive on the scene of the attack. He told reporters that the carnage was "unspeakable" and that those who had not been injured had "pure panic" on their faces.
It was deadliest mass shooting in the United States since a gunman opened fire at an elementary school three years ago, leaving 26 children and adults dead. It was the 355th time this year in the U.S. in which four or more people have been shot in a single incident.
Farook's brother-in-law, Farhan Khan, said he was bewildered by news of the shooting.
"Why would he do that?" Khan asked. "Why would he do something like this? I have absolutely no idea. I am in shock myself."
The massacre stunned the U.S. Muslim community, whose leaders say they are as heartbroken and horrified as everyone else.
"There's a lot of anxiety among American Muslims, because we have seen it in the past." said Nihad Awad, head of the Council on American-Islamic Relations. "We've seen what jumping to conclusions means and how it impacted our lives."
Awad said he spent part of Thursday answering telephone calls from parents worried about whether it was safe to send their children to school because of a possible backlash.
Hussam Ayloush, executive director of the Council on American-Islamic Relations in Los Angeles, appealed to Americans to not jump to conclusions about the motives behind the rampage.
"Is it work? Is it rage-related? Is it mental illness? Is it extreme ideology?" Ayloush said. "We just don't know.
"We unequivocally condemn the horrific act that happened today," Ayloush told reporters. "We stand in solidarity in repudiating any possible ideology or mindset that could have led to such [a] horrific act."
A third person was arrested while trying to flee the shootout, but Burguan said that it was unclear what he was doing at the scene and that he was not believed to be a shooter.
This shooting came less than a week after a gunman killed three people and wounded nine in a rampage at a Planned Parenthood women's health clinic in Colorado Springs, Colorado.
A gunman killed nine people at a college in Oregon in October, and a white gunman killed nine black churchgoers in South Carolina in June.