SAN BERNARDINO, California -- It started at a holiday party -- perhaps with a slight or a testy exchange, something that prompted Syed Rizwan Farook to storm off angrily. It ended in a bloodbath with 14 people dead and 17 more wounded -- the deadliest mass shooting in the United States since Sandy Hook.
At its center, a couple, Farook and Tashfeen Malik.
Dressed in black, carrying semi automatics, they unleashed a massacre Wednesday at the Inland Regional Center, a facility for the developmentally disabled in San Bernardino, California.
That was their first brazen act. Then they led police on a chase. Farook fired while Malik drove.
They died in a hail of bullets when they tried to take on 21 officers.
Now comes the challenging part: What was their motive? Surely it couldn't just have been anger at a party. The level of attention speaks to something much more meticulously planned.
But police don't yet know.
The couple didn't leave behind a note at Inland Regional. But they did stash three explosive devices -- rigged to a remote-controlled toy car -- that didn't go off.
The mass shooting
It was around 11:00 a.m. when Farook and Malik opened fire.
A text message landed in Terry Pettit's phone from his daughter, who was inside Inland Regional.
"Shooting at my work. People shot," she wrote. "Pray for us. I am locked in an office."
Denise Peraza was also inside the center when she was shot in the back. She called her sister Stephanie Baldwin, thinking it might be time to say goodbye.
"As soon as the gunfire started, everyone dropped to the floor and they were underneath desks, and she was trying to shield herself with a chair, along with a man next to her," Baldwin told CNN affiliate KABC. "Then, all of a sudden, she said she just felt (the bullet) going through her back."
"I just want to tell you that I love you," Peraza told Stephanie Baldwin over the phone through tears.
Peraza survived. She is in a hospital and is expected to recover.
Police have not released the names of those who died.
Within minutes, troops of officers stormed the building searching for an active shooter. They counted the dead -- and shuttled the wounded out to triage.
"We had to come out with our hands up and be escorted across the street to the golf course," a woman who works at the center told KCAL/KCBS.
"We stood there for hours, hours witnessing clothing of deceased ones on the street, people crying, co-workers crying, us wanting to get to our children."
But Farook and Malik slipped away in a black SUV.
Not for long. Acting on information that quickly pointed police to Farook, they went to his home in Redlands with a search warrant.
A black SUV drove by them. Slowly at first, then it sped away.
A police car took up pursuit, as the SUV raced back in the direction of San Bernardino. While Malik drove, Farook opened fire out of the vehicle.
Some 21 officers returned fire. When the SUV came to a halt, it was riddled with bullet holes. The couple inside was dead; their bodies found dressed in "assault-style clothing," San Bernardino Police Chief Jarrod Burguan said. "Dark kind of tactical gear."
One officer was wounded, but his injuries were not life-threatening, Burguan said.
In the chaos, police encountered a third person who was running away. "We do not know if they were involved," Burguan said. "We have that person detained."
But they feel confident that there were only two shooters -- Farook and Malik.
This is notable given that, while there have been many mass shootings, it's extremely rare when they involve more than one shooter. Only two of the 28 deadliest shootings since 1949 in the United States have had more than one shooter.
Two .223 caliber rifles were in the car with them, along with two pistols.
They were legally purchased, police said.
Two handguns traced back to Farook, an official said. He bought them three to four years ago.
Someone else bought the two rifles, possibly a former roommate -- also legally three or four years ago. That person isn't believed to have anything to do with the shootings, the official said.
"I think that what we have seen and how they were equipped, there had to be some kind of planning in this," Burguan said.
Back at Inland Regional, there was still danger to be dealt with -- three explosives the pair had left behind.
They were "pipe bomb type design," Burguan said. Police secured them and remotely detonated them.
The explosives had been rigged to a remote control for a toy car, an official said. That remote was found inside the SUV. And in the vehicle was another pipe-like device, but it was not an explosive, Burguan said.
In Redlands, officers sent in a robot to check Farook's residence, which they held surrounded into the night.
Farook, an American citizen, was an environmental health specialist with the San Bernardino County health department, which was hosting the holiday party at Inland Regional.
He had worked there for five years.
In an online profile, he described himself as a "22-year-old Muslim Male living in USA/California/riverside. Religious but modern family of 4, with 2 girls and 2 boys."
He "enjoys working on vintage and modern cars, reads religious books, enjoys eating out sometimes. Enjoys travelling and just hanging out in the back yard doing target practice with his younger sister and friends," his profile read.
Farook's brother-in-law Farhan Khan was crushed at the news.
"I have no idea why he would he do something like this. I have absolutely no idea. I am in shock myself," Khan said. "I don't have words to express how sad and how devastated I am."
Khan said he last talked to Farook a week ago. Farook's family had tried to reach him Wednesday but could not all day.
CNN's Alberto Moya, Tina Burnside, Dave Alsup, Andy Rose, Evan Perez, Barbara Starr, Pamela Brown, Deborah Feyerick, Michael Martinez, Joshua Berlinger, Ashley Fantz, Joshua Gaynor, Jason Hanna, John Newsome, Stella Chan, Nadia Kounang and Catherine E. Shoichet contributed to this report.