LOS ANGELES—Three members of a street gang have pleaded guilty on the eve of trial to their roles in the robbery and fatal shooting of Los Angeles Police Department Officer Fernando Arroyos who was gunned down in January 2022 while off-duty and house-hunting with his girlfriend, officials announced on July 14.

The defendants—Luis Alfredo de la Rosa Rios, 29; Ernesto Cisneros, 24; and Jesse Contreras, 35—each pleaded guilty on July 13 to one count of conspiracy to violate the Racketeer Influenced and Corrupt Organizations (RICO) Act, which allows the government to punish criminals associated with gangs and crime organizations, according to the U.S. Department of Justice.

The trial was set to begin on July 11 in Los Angeles federal court for the three. A fourth defendant, Haylee Marie Grisham, 20, a gang associate who was Mr. Rios’s girlfriend, entered her guilty plea to the charge in April.

According to plea agreements filed Thursday, on the night of Jan. 10, 2022, the defendants were driving around the Florence-Firestone area of South Los Angeles looking for someone to rob when they came upon Mr. Arroyos, who was wearing gold chains around his neck.

Mr. Rios and Mr. Cisneros exited a black pickup and confronted Arroyos, a three-year veteran of the Los Angeles Police Department, and his girlfriend as they were searching for a home to purchase in the area.

The two gang members pointed guns at the victims and removed property from both, including a wallet and two silver chains from Mr. Arroyos’ neck. At some point after Mr. Cisneros removed Mr. Arroyos’ chains, the off-duty officer and the gang members exchanged gunfire, according to court documents.

Mr. Arroyos sustained a single gunshot wound, ran from the area, and collapsed in an alley as the two defendants left the scene in the truck.

Funeral services for LAPD Officer Fernando Arroyos, who was shot and killed in January, takes place at Forest Lawn Hollywood Hills in Los Angeles, on Feb. 2, 2022. (Irfan Khan/Getty Images)

Responding law officers found bystanders performing CPR on Mr. Arroyos. The officers loaded Mr. Arroyos into a patrol car and took him to a hospital, where he was pronounced dead.

Detectives retrieved a loaded handgun from the scene belonging to one of the suspects, and the Los Angeles County Sheriff’s Department received a report of a man suffering from a gunshot wound in the area of Junction Street and East 60th Street about a mile-and-a-half from the site of the shooting.

Investigators later determined the wounded man was Mr. Cisneros, who had sustained the gunshot during the shootout with Mr. Arroyos. Mr. Contreras was also found in the area hiding inside the garage of his residence in the 5900 block of Junction Street, where a second handgun was retrieved.

Mr. Rios and Ms. Grisham—who were then a couple—were later found and taken into custody at their home.

Mr. Rios and Mr. Contreras further admitted in their plea agreements to committing armed robberies against two victims outside a bar in the Florence-Firestone area earlier the same day.

The three gang members pleaded guilty before U.S. District Judge Percy Anderson, who scheduled sentencing hearings on Sept. 25 for Mr. Rios and Mr. Contreras, and an Oct. 16 sentencing hearing for Mr. Cisneros. As a result of their guilty pleas, each defendant faces up to life in federal prison. Prosecutors have agreed to seek terms of between 35 and 50 years in prison for Mr. Rios and Mr. Cisneros, and a term of 35 years for Mr. Contreras.

Ms. Grisham’s sentencing hearing is scheduled for Sept. 11.

Fernando Arroyos. (Courtesy of Los Angeles Police Department)

“This case starkly illustrates the devastating impact of gangs on our community,” U.S. Attorney Martin Estrada said in a statement.

“A young man who had achieved great success and returned to serve his community as a Los Angeles Police officer was murdered while shopping for a home in the city he took an oath to protect,” said Mr. Estrada. “These senseless tragedies are repeated too often. Our work in this case sends a message that we will aggressively prosecute violence against our community.”

Former Sheriff Alex Villanueva indicated at the time of the arrests that he directed detectives to take the case to federal prosecutors because of his opposition to District Attorney George Gascón’s decision to no longer pursue sentencing enhancements in gang cases.

Those enhancements in a murder case can mean the difference between a life term with the possibility of parole and never being released from prison.