Four of the nine people killed during a biker gang brawl in Waco, Texas, last spring were shot by the same caliber of rifle used by police at the scene, new evidence obtained by The Associated Press shows.

The detail is the latest uncovered that may indicate whether any of the officers responding to the May 17 melee were responsible for the fatalities. At least 20 others were wounded.

Related: 106 People Indicted in Waco Biker Brawl That Killed Nine

Cops arrested more than 170 people amid the chaotic clash at the Twin Peaks sports bar — and last month, a Texas grand jury returned indictments against 106 of them. They face charges of engaging in organized criminal activity, with the underlying offenses being murder and aggravated assault.

The ballistics reports reviewed by the AP indicate that four of those killed were struck by bullets from .223-caliber rifles — the only type of weapon fired by police. It's not clear if any bikers had weapons of a similar caliber, the AP reported. Twelve long guns were among the weapons recovered from the scene.

Waco Police Chief Brent Stroman said in June that cops fired 12 rounds from their rifles.

While all four victims had wounds from .223-caliber rounds, two of them were also hit by other kinds of bullets, according to the evidence.

Police have released few details about how the violence unfolded. Police and the district attorney's office would not comment on the ballistics reports to the AP.

Investigators have said officers had to shoot their weapons in self-defense after the violence spilled out of the restaurant and escalated from fists to firearms. The dead were members of either the rival Bandidos or Cossacks gangs, police said.

Many witnesses — from the bikers to the Twin Peaks waitresses — say the shooting began after a Bandidos member hit a prospective Cossacks member with his bike, according to evidence made public last month in September.