WACO, Texas — Police were on alert Monday for any retaliatory attacks after a chaotic shootout between rival biker gangs left nine people dead and at least 18 more wounded outside a Texas restaurant.

Waco police Sgt. W. Patrick Swanton said authorities had received threats against law enforcement “throughout the night” from biker groups and stood ready to confront any more violence resulting from Sunday’s gunfire.

“We have a contingency plan to deal with those individuals if they try to cause trouble here,” Swanton said at a news conference.

About 170 people were arrested on charges of engaging in organized crime. Earlier, Swanton said 192 people had been arrested but later revised that number downward.

The shootout erupted shortly after noon at a busy shopping center along Interstate 35 where members of at least five rival gangs had gathered for a meeting, Swanton said.

Preliminary findings indicate a dispute broke out in a bathroom and escalated to include knives and guns. The fight eventually spilled into the restaurant parking lot.

“I was amazed that we didn’t have innocent civilians killed or injured,” Swanton said.

The interior of the restaurant was littered with bullet casings, knives, bodies and pools of blood, he said.

Authorities were processing the evidence at the scene, 95 miles south of Dallas. About 150 to 200 bikers were inside during the shootout.

Parts of downtown Waco were locked down, and officials stopped and questioned motorcycle riders. Agents from the FBI and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives were assisting local and state authorities.

Police and the operators of Twin Peaks — a national chain that features waitresses in revealing uniforms — were aware of the meeting in advance and at least 12 Waco officers in addition to state troopers were outside the restaurant when the fight began, Swanton said.

Officers shot armed bikers, he said.

It was not immediately clear whether any of the nine dead were killed by police. The identities of the dead have yet to be made public.

The Texas Alcoholic Beverage Commission closed the Twin Peaks location for a week out of fear of more violence, Swanton said.

A statement released Sunday night on behalf of Jay Patel, operating partner for the Waco franchise, said his management team was having “positive communications with the police.”

But Swanton said the management has not cooperated with authorities in addressing concerns about the gangs and called Patel’s statement a “fabrication.”

Rick Van Warner, a spokesman for the Dallas-based corporate franchisor, said the company is “seriously considering revoking” the Waco location’s franchise agreement.

McLennan County Sheriff Parnell McNamara, whose office is involved in the investigation, said all nine who were killed were members of the Bandidos or Cossacks gangs.

In a 2014 gang threat assessment, the Texas Department of Public Safety classified the Bandidos as a “Tier 2” threat, the second highest. Other groups in that tier included the Bloods, Crips and Aryan Brotherhood of Texas.

The Bandidos, formed in the 1960s, are involved in trafficking cocaine, marijuana and methamphetamine, according to the U.S. Department of Justice.

El Paso authorities in 2012 said several Bandido members were involved in an assault and robbery at two bars, according to the assessment. State arrest warrants were issued for aggravated assault with a deadly weapon, engaging in organized crime and other crimes, and six of the suspects were arrested.

The Bandidos conduct their activities as covertly as possible to avoid publicity, according to the DPS assessment. Members are not covert, however, about making their presence known by wearing their colors and insignia, and riding in large groups.

The Texas assessment does not mention the Cossacks.

There’s at least one previously documented instance of violence between the two groups. In November 2013, a 46-year-old from Abilene who police say was the leader of a West Texas Bandidos chapter was charged in the stabbings of two members of the Cossacks club.