A Dallas police officer was arrested Thursday on two counts of capital murder after a man told investigators that he kidnapped and killed two people at the officer's instruction in 2017, authorities said.

Bryan Riser, a 13-year veteran of the force, was taken into custody Thursday morning and taken to the Dallas County Jail for processing, according to a statement from the police department. A lawyer for him couldn't immediately be identified.

Riser was arrested in the unconnected killings of Liza Saenz, 31, and Albert Douglas, 61, after a man came forward in August 2019 and told police he had kidnapped and killed them at Riser's direction, police Chief Eddie Garcia said during a news conference. He said investigators don't know the motives for the killings, but that they were not related to Riser's police work.

Garcia did not explain why Riser was arrested more than a year after the witness came forward, and police did not immediately respond to questions about the timing or whether the person who implicated Riser has also been charged. The chief stressed that his homicide division and the FBI were still investigating.

Saenz's body was pulled from the Trinity River on March 10, 2017, with several bullet wounds, the chief said. Douglas was reported missing that year and his body hasn't been found.

The murder charges are not the officer's first alleged crimes. In May 2017, Riser faced a domestic violence charge for allegedly assaulting and injuring an ex-girlfriend. It wasn't immediately clear how that case was resolved, and Garcia declined to comment on it Thursday.

Riser joined the department in 2008 and was working as a patrol officer before his arrest. Garcia acknowledged that Riser had been patrolling Dallas while under investigation for murder and said the department is reviewing his arrests.

Riser has been put on administrative leave pending the outcome of an internal affairs investigation. Garcia said "we're going to expedite our process" toward his firing.

"We will not allow anyone to tarnish this badge," the chief said.

Riser had not been booked into the jail as of early Thursday afternoon, a sheriff's spokesman said.