SANTA MONICA, Calif. — Police investigating why a heavily armed gunman plotted a rampage that killed four people and wounded several others were focused Saturday on how the violence began: directed at his own family.

What started as domestic violence led to a chaotic street shooting spree and ended less than 15 minutes later in a college library where the gunman was killed Friday by police as students studying for finals ran for cover or hunkered down to avoid whizzing bullets.

Investigators were looking at family connections to find a motive because the killer’s father and brother were the first victims, said an official briefed on the probe who requested anonymity because he wasn’t authorized to speak publicly.

The killer, who died a day shy of his 24th birthday, was connected to the home that went up in flames after the first shootings, said Police Chief Jacqueline Seabrooks. She refused to elaborate or name the suspect because a surviving family member was out of the country and couldn’t immediately be notified.

At an afternoon news conference next to the weapons and ammo found at multiple crime scenes, Seabrooks said the “cowardly murderer” planned the attack and was capable of firing 1,300 rounds.

“Any time someone puts on a vest, of some sort, comes out with a bag full of loaded magazines, has an extra receiver, has a handgun and has a semi- automatic rifle, carjacks folks, goes to a college, kills more people and has to be neutralized at the hands of the police, I would say that that’s premeditated,” she said.

The killer had a run-in with police seven years ago, but Seabrooks wouldn’t offer more details because he was a juvenile at the time.

His father, Samir Zawahri, 55, brought his family to the neighborhood of small homes and apartment buildings tucked up against Interstate 10 in the mid-1990s, according to property records.

Not long after arriving on Yorkshire Avenue, the couple went through a difficult divorce and split custody of their two boys, said Thomas O’Rourke, a neighbor.

“It was not an easy breakup,” O’Rourke said. “It was a bitter divorce.”

When the sons got older, one went to live with his mother while the other stayed with the father.

“The father was a very nice gentleman,” O’Rourke said. “But the boys just kind of kept to themselves. Didn’t really socialize with any of the neighbors.”

SWAT team officers searched the mother’s Los Angeles apartment Friday night and officers interviewed neighbors about the son who lived with her, said Beverly Meadows who lives in the adjoining unit.

Public records show that Meadows’ neighbor is Randa Abdou, 54, the ex-wife of Zawahri and former co-owner of the house where the first shooting took place.

The mother was out of the country visiting relatives and wasn’t expected home for another week, Meadows said. It wasn’t clear if the son who lived with Abdou was a victim or the suspected gunman.

The gunman was enrolled at Santa Monica College in 2010, Seabrooks said.

After neighbors watched in shock as he shot at his father’s house and it went up in flames, he opened fire on a woman driving by, wounding her, and then carjacked another woman.

He directed her to drive to the college, ordering her stop along the way to shoot at a city bus and people on the street. Two people on the bus were injured.

Police had received multiple 911 calls by the time the mayhem shifted to the college, a two-year school with about 34,000 students located among homes and strip malls more than a mile inland from the city’s famous pier, promenade and expansive, sandy beaches.

On campus, he opened fired on a Ford Explorer, killing the driver, who plowed through a brick wall into a faculty parking lot. A female passenger was gravely wounded.

The driver was identified as Carlos Navarro Franco, 68, a campus employee.

Bursar’s office employee Joe Orcutt heard gunshots and went to see what happened in the parking lot. He saw the Explorer in the brick wall and was looking for the shooter when, suddenly, there he was 30 feet away firing at people like it was target practice.

“He’s just standing there, like he’s modeling for some ammo magazine,” Orcutt said. “He’s not emotional. I don’t know if he’s zoned out or focused, he was very calm just standing there, panning around.”

As a bullet whizzed by, Orcutt jumped out of the way.

The gunman then moved on foot across campus, firing away. Students were seen leaping out windows of a classroom building and running for their lives. Others locked themselves behind doors or bolted out emergency exits.

At some point, he dropped an Adidas duffel bag loaded with ammunition magazines, boxes of bullets and a .44 revolver. Police also found a small cache of ammunition in a room in the burned-out house.

Trena Johnson, who works in the dean’s office, heard gunshots and looked out the window and saw a man in black with a “very large gun” shoot a woman in the head outside the library.

“When I saw her shot in the head and she fell to the ground we ran out the back door,” Johnson said. “I haven’t been able to stop shaking.”

Surveillance photos displayed at the news conference showed the gunman in black strolling past a cart of books into the library with an assault-style rifle by his side.

Vincent Zhang, an economics major, was studying in the library when he heard a female scream, “No, no. Please no.” Zhang ran out of the emergency exit while others took cover in what Seabrooks called a “safe room,” barricaded behind a door.

“They stacked items found in the safe room against the door, hunkered down and avoided shots fired through the drywall at them while they were in that room,” she said.

The shooter fired at least 70 rounds in the library. Miraculously, no one was injured until two Santa Monica police officers and a campus cop arrived and took out the shooter. He was struck multiple times.