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Montebello police are looking for a "documented gang member" suspected of killing his estranged wife over the weekend, and a female alleged accomplice has been arrested.

Christopher Michael Diaz, 26, is believed to be armed and dangerous. Police are asking for help finding him.

Roxann Acosta, 28, was found killed just before 10 p.m. Sunday, according to a Montebello Police Department flier.

Officers were called to a report of shots fired in the 1200 block of West Victoria Avenue in Montebello, and they found the victim dead from a gunshot wound. Witnesses identified Diaz, Acosta's husband of about a year, as the shooter, police Sgt. Marc Marty said.

A female suspected of being an accomplice has been arrested, Marty said. Her name was not being released.

Acosta was killed in Diaz's mother’s apartment, according to Marty. Diaz used an “unknown type firearm” to kill Acosta, who was pronounced dead at the scene, police said.

A relative of Acosta told the San Gabriel Valley Tribune that the victim was 8 weeks pregnant and had filed for divorce.

Friends of Acosta at the scene of the shooting also said Acosta was 8 weeks pregnant, and said the victim leaves behind a daughter and two sons. Marty said investigators are waiting for the medical examiner to confirm if she was pregnant.

The estranged couple spent the day together and then a verbal argument led to the shooting, the sergeant said.

"We don't have any idea why he killed her," Marty said.

A vigil held for Acosta on Monday night was heavily attended by friends and family; candles, balloons and flowers were left outside the apartment building. "Rest in Peace Roxy" was spray-painted on the sidewalk.

Many friends recalled her warm smile, and Christina Franco, her co-worker at Homeboy Industries, said it lit up the entire workplace and was the first thing you noticed when you walked into the room.

“As beautiful as Roxy is on the outside is as beautiful as she was on the inside,” Franco told KTLA.

Another co-worker, Jimmy Campa, said Acosta was "a light at Homeboy Industries" and always "generous with her smile and her gift of friendship.”

Speaking at the vigil, Alicia Acosta, the victim's mother, described her son-in-law as a "monster" and said she never trusted him.

"I always feared him," she said. "That's why he wasn't welcome into the family."

Diaz is described Hispanic, as 5 feet 5 inches tall and 145 pounds, with brown hair and brown eyes.

He had an extensive arrest record, including for assault with a deadly weapon on a police officer, Marty said. It's not clear if Diaz was convicted.

Friends gathered Tuesday at the Montebello apartment building where Acosta died said they were in shock. They urged Diaz to turn himself in.

"He's not going to get away with it. He should know that," Marilyn Morones said. "I don't know how a man could do that to a woman, and then with a baby in the stomach."

Acosta loved Diaz but was trying to leave her husband, who was "obsessive" and "dangerous," another friend, Tiff Noriega, said.

Friends at the vigil said Acosta had filed for divorce and wanted to move on, but Diaz didn't want to.

A GoFundMe page has been set up to pay for Acosta’s funeral.

Correction: An earlier version of this article incorrectly described the apartment where Acosta was killed. The story has been updated.

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