An Arizona prisoner serving 16 life sentences killed three other inmates at a state prison in Tucson, according to officials.

Ricky Wassenaar is accused of killing Saul Alvarez, Thorne Harnage and Donald Lashley during the altercation on Friday at the Arizona State Prison Cimmaron Unit, according to KOLD.

Wassenaar is the only suspect in the incident, the outlet reported. Prison officials say he acted with “intent to harm.”

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Alvarez was behind bars for a Maricopa County murder, Harnage was convicted in Pima County of sexual conduct with a minor and Lashley was serving a sentence for child molestation in Pima County.

Wassenaar was already serving 16 life sentences for his role in a prison standoff in 2004, according to KOLD. He already had a criminal history before the standoff.

He and another man were convicted of taking two guards hostage for 15 days at Lewis Prison Complex in Buckeye, Arizona. It was one of the longest prison hostage situations in U.S. history.

Wassenaar allegedly used a kitchen guard’s uniform to trick another guard into letting him into a prison watchtower. He obtained the uniform by overpowering an officer with a handmade weapon.

The Arizona Department of Corrections said Wassenaar was moved to a prison in Florence after Friday’s inmate deaths. The incident remains under investigation.

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“ADCRR’s Office of the Inspector General has begun an initial criminal investigation related to the inmate’s deaths,” prison officials said in a statement. “All inmate deaths are investigated in consultation with the county medical examiner’s office. The ADCRR takes situations involving violence seriously and will pursue prosecution against any suspects believed to be involved in the case.”

Visitations at the prison were canceled for the rest of the day on Friday following the altercation.