Police arrested five men Thursday in connection with the deadly Halloween shooting at an Orinda house party that killed five people, while revealing that two of the victims were armed with guns when they were slain.

Four suspects were booked on suspicion of murder and conspiracy and were being held without bail: Lebraun Tyree Wallace, 28, of San Mateo; Jaquez Deshawn Sweeney, 20, and Jason Iles, 20, both of Marin City; and Shamron Joshua Mitchell, 30, of Antioch.

The fifth, Devin Isiah Williamson, 21, of Vallejo was arrested on suspicion of being an accessory, officials said. He’s being held on $500,000 bond.

A source with knowledge of the investigation told The Chronicle that the suspects showed up at the party with Javlin County, 29, who was killed in the gunfire.

How the shooting unfolded remains a mystery — at least to the public. After two weeks, officials with the Contra Costa County Sheriff’s Office remained tight-lipped about what they believe led to the shooting and whether victims were targeted. They did not say whether all four suspects charged with murder and conspiracy fired weapons — or whether all were present during the Orinda party.

In a news release Thursday announcing the arrests, Sheriff David Livingston said, “Evidence shows two of the deceased persons inside the house were themselves armed, which may have played a role in this tragedy.”

Livingston did not name those two or say whether they fired their weapons.

The arrests were made in San Mateo, Marin City, Vallejo and Antioch and came two weeks after gunfire erupted inside a Lucille Way house crowded with an estimated 100 partygoers. The bloodshed left residents of the tranquil community in shock.

“We certainly hope this gives some sense to residents and people around the community that the Contra Costa County Sheriff’s Office is working hard on this case,” Orinda Mayor Inga Miller said Thursday.

Attorneys for the suspects were not immediately available for comment. The four facing murder charges have ties to Marin County. Two, Mitchell and Wallace, have criminal records.

Mitchell listed on his LinkedIn account that he attended Sir Francis Drake High School in San Anselmo.

A decade ago, according to media reports, Mitchell pleaded guilty to a gang-related assault charge involving a Sept. 1, 2008, incident in which prosecutors said he and three other people beat six members of a family during a street festival in the area of Drake Avenue. Prosecutors said at the time that the attackers were acting on behalf of a reputed street gang in the public housing complex on that street.

The plea deal allowed Mitchell to avoid a potential maximum term of 19 years, reports said.

Wallace had a run-in with law enforcement in Marin County three years ago. The California Highway Patrol arrested him on suspicion of possessing weapons after a high-speed pursuit that started in Mill Valley and ended in Marin City.

Sweeney played sports and attended two Marin County high schools, according to his social media accounts. He played football for Redwood High School in Larkspur his sophomore year and then transferred to Sir Francis Drake High School, where he played basketball.

Iles played basketball for Redwood High School in 2014, and Williamson played basketball at Hercules High School.

County, the man killed at the party who was said to have arrived with the suspects, had served time in prison. He was released in March 2018 after serving a two-year sentence for possession of stolen property, according to the California Department of Corrections and Rehabilitation. Records show that he lived in Richmond but frequented the same Marin City community as some of the suspects.

Before serving the most recent prison sentence, County had done state prison stints for robbery, grand theft, and drug charges, the records show.

Authorities identified the other four people killed on Oct. 31 as Tiyon Farley, 22, of Antioch; Omar Taylor, 24, of Pittsburg; Raymon Hill Jr., 23, of San Francisco and Oakland; and Oshiana Thompkins, 19, of Vallejo and Hercules. At least four other people were injured in the shooting.

The San Francisco Police Department’s gang task force was assisting in the investigation and looking into whether the Orinda case was linked to a quadruple gang killing in Hayes Valley in 2015. Farley’s brother, Lee Farley Jr., is charged in that case. Contra Costa County investigators, however, have not commented on the possible link.

Farley was a childhood friend of Hill, who was paroled in July after serving a sentence for assault with a firearm in a Potrero Hill drive-by shooting, to court records show.

The night of the Orinda shooting, police responded to the gunfire at 10:48 p.m. and found a chaotic scene as party attendees ran and drove down the narrow roads south of the city’s BART station and Highway 24. Police said the partygoers were mostly in their teens and 20s and had attended the event that was promoted on social media as a “mansion party.”

Residents and victims’ families have criticized police for their response times. Neighbors called to complain about the noise around 9:19 p.m. and 10:25 p.m., but officers didn’t arrive until after the gunshots.

The Orinda Police Department’s two patrol units on duty that night were assisting Lafayette police in a home invasion robbery that led them to Oakland.

The violence drew outrage from residents in Orinda who said they had complained numerous times over the last year to the city and Airbnb, through which the house was rented, about unruly parties there. Airbnb has responded that it is taking steps to ban party houses on its online platform, and the city has placed a ban on some short-term rentals.

Bay Area attorney John Burris said he is representing the families of Farley, Thompson and County and said he’s working to determine whether there’s a question of liability in the incident. An attorney for Hill’s family said they already filed a lawsuit against Airbnb.

San Francisco Chronicle staff writer Alejandro Serrano contributed to this report.

Evan Sernoffsky and Matthias Gafni are San Francisco Chronicle staff writers. Email: esernoffsky@sfchronicle.com, matthias.gafni@sfchronicle.com Twitter: @evansernoffsky, @mgafni