It was the worst shooting ever inside a Nevada casino.

Denise Massey, 48, was gambling with her fiancée on the first floor of Harrah’s when she noticed 20 to 30 bikers suddenly converge.

“Next thing you know you just hear ‘Bam, bam, bam,'” she said. “All of a sudden they’re running and just shooting at each other.”

Police said 60 to 70 people, armed with guns and knives, were involved in the violence inside the entrance to Harrah’s casino. They had arrested one person and recovered a handgun, said Sgt. Chris Darcy, spokesman for the Las Vegas Metropolitan Police Department, which patrols the Colorado River gambling resort 80 miles southeast of Las Vegas.

“We have hundreds of people that we’re attempting to locate and interview,” he said.

Darcy said rival motorcycle gangs were involved but wouldn’t say which ones. He said all three of the dead were bikers.

Authorities also warned of the danger for further violence.

“The rumors of retaliation have already hit the street,” said Larry Tunforss, spokesman for the fire department in Bullhead City, Ariz., which is across the river from Laughlin. “Our police department is beefing up intensely. There’s a definite concern of further incidents.”

SWAT teams and law enforcement officers from across Arizona and Nevada were called in.

The shooting occurred during the 20th annual River Run, a weekend gathering of motorcycle clubs that brought up to 80,000 people into Laughlin. The resort town of about 8,000 residents has nine casinos and attracts about 5 million visitors a year.

Authorities locked down the city after the violence erupted shortly after 2 a.m., temporarily closing bridges over the Colorado River. The routes were reopened later in the morning.

Nevada state archivist Guy Rocha said it was the worst shooting ever inside a Nevada casino.

“In every case that’s been described to me there was no more than one death. This is a threshold,” he said.

Aniko Kegyulics was sitting at a casino bar with her boyfriend when she heard yelling and gunfire.

“There were so many gunshots everything turned gray. Everybody fell to the floor. It was crazy,” she said.

She passed a body covered with a bloody sheet when police allowed people inside to leave at about 8 a.m., she said.

Chris Welton, 42, of Carson City, had been playing poker.

“Shots just started going off,” he said. “There were dead bodies. Pretty nasty. There was a guy about 10 feet from me. He was dead. His buddy was trying to revive him.”

University Medical Center in Las Vegas was treating five patients, three of whom were in serious condition with gunshot and stab wounds, nursing supervisor Louise Burns said. Of the other two, one was in critical condition with a skull fracture and the other was in fair condition with gunshot and stab wounds.

Western Arizona Regional Medical Center in Bullhead City was treating seven fight victims. All were in fair condition.

Authorities were also investigating the death of a biker whose body was found along Interstate 40, a main route between Laughlin and Southern California.

The man’s body was found near Ludlow, a small California desert town about 60 miles from the Arizona border. Preliminary reports indicate the man, from San Diego, had been shot while riding his bike and was a member of a motorcycle club, San Bernardino County Sheriff’s spokesman Chip Patterson said.

It was not immediately known if the death was related to the casino shooting.

Traffic in both directions was being diverted off the interstate as sheriff’s deputies investigated, the California Highway Patrol reported.