The semiautomatic rifle used by a convicted felon in a gun battle with police off the 215 Freeway on Monday, killing CHP Officer Andre Moye and wounding two of his colleagues, was a “ghost gun” that cannot be traced by authorities, according to law enforcement sources.

Investigators have publicly described the gun used only as a rifle, but a law enforcement source with knowledge of the investigation told The Times that Aaron Luther used an “AR-15-style rifle” in the fatal battle. The sources spoke on condition of anonymity in order to discuss the case candidly.

Ghost guns are unserialized weapons that are manufactured from parts that can be ordered through the mail or are machined parts acquired from underground makers.

The Bureau of Alcohol, Tobacco, Firearms and Explosives has been unable to trace the weapon used by Luther, two law enforcement sources with knowledge of the investigation said.

Luther, a convicted felon with an extensive criminal background, would not have been allowed to possess a firearm under California law because of his criminal record.

Authorities are seeing a proliferation of such untraceable weapons.

“About a third of all firearms seized in Southern California now are unserialized, and that is expected to grow,” said Ginger Colbrun, a spokeswoman for the Los Angeles regional ATF office.

Those with lengthy criminal histories, according to law enforcement officials, usually buy their weapons on the black market of underground weapons sales. In one raid last year, authorities recovered 45 ghost guns after a six-month undercover operation in Hollywood.

In that case, some of the weapons made by a gang were assault weapons.

An AR-15 like the one Luther possessed shoots small, high-velocity bullets and can be modified to use large-capacity magazines. The .223-caliber rounds often shatter inside victims’ bodies, creating more devastating injuries than wounds typically left by larger but lower-velocity handgun rounds.

A source familiar with the weapon recovered from Luther after he was killed said that the magazine probably had a capacity of 30 rounds and that he was able to fire a large number of rounds before Riverside police and sheriff’s deputies joined the gun battle and cut him down on the 215 Freeway.

Investigators are still trying to determine how Luther came to possess the weapon.

Court records show Luther pleaded guilty to one count of attempted murder and two counts of burglary in Los Angeles County in 1994. He was sentenced to 12 years in prison and was granted parole in 2004, according to the California Department of Corrections and Rehabilitation.

The Beaumont resident also had been convicted of disturbing the peace, vandalism, battery, stalking, unlawful possession of a firearm, assault with a deadly weapon and corporal injury on a spouse, officials said.

Riverside Police Chief Sergio Diaz said the gun battle that mortally injured Moye, 34, was “long and horrific,” leaving investigators with multiple crime scenes to examine. Moye, who had been with the California Highway Patrol for almost three years, was airlifted to a hospital in Moreno Valley after the shootout, where he was pronounced dead. The two other CHP officers wounded in the shootout are expected to recover, said Inland Division Chief Bill Dance.

The deadly encounter began at 5:35 p.m. when Moye stopped Luther, who was driving a white GMC pickup, at the Eastridge Avenue/Eucalyptus Avenue offramp. At some point during the traffic stop — for reasons not yet known — Moye decided to impound the man’s vehicle and called for a tow truck, said CHP Inland Division Assistant Chief Scott Parker.

It was not immediately clear where Luther was heading when he was stopped. While Moye was filling out paperwork, the man got a rifle from his truck and started shooting at the officer, Parker said.

“We don’t know why” the shooting began, Riverside police spokesman Ryan Railsback said. “That is all going to be part of this lengthy investigation.”

Moye returned fire, and even though he had been shot, he was able to radio for help. Three other CHP officers soon arrived, followed by three deputies from the Riverside County Sheriff’s Department and at least one officer from the Riverside Police Department, authorities said.

Ghost guns are legal for those allowed to own firearms. But California requires that anyone building a weapon apply for a unique serial number with the state’s Department of Justice and that number be put on the firearm. The weapon must comply with California’s laws governing firearms.

Police gun experts say that those forbidden from purchasing a gun can still buy the partial lower receiver, known as an “80% lower,” and then buy the other parts of the weapons.

In 2013, John Zawahri killed five people in the Santa Monica area using an AR-15-style rifle, which he built from a partially manufactured lower receiver that did not have a serial number and therefore did not have to be registered. Police said the parts needed to assemble a working rifle, including magazines of ammunition, were shipped in from out of state.

When the Los Angeles Police Department and ATF netted 45 ghost guns from a gang last year, a top ATF agent described the growing problem of the weapons.

“Criminals are making their own weapons because they cannot buy them legally … or they are paying other people to make those guns for them to get around the gun laws,” said Bill McMullan, then the special agent in charge of the ATF’s L.A. field division.

A serial number is issued only to the lower receiver of a weapon.

Some lawmakers in California are seeking to require a background check for all gun parts to stifle the trade in ghost guns.