Denver's sheriff says releasing an inmate from its downtown jail over the weekend was an unacceptable mistake.

Ivan Zamarripa-Castaneda is a suspect in a deadly hit-and-run crash on March 3 that claimed the life of 57-year-old truck driver John Anderson. Zamarripa-Castaneda was charged with vehicular homicide-DUI in connection to that wreck.

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Ivan Zamarripa-Castaneda (Photo: DA's office)

He's also wanted by immigration officials. Zamarripa-Castaneda posted his $25,000 bond at 8 a.m. Saturday.

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He left jail at 5:28 that evening, but ICE didn't get the heads up until an hour later at 6:33 p.m.

As a consequence, ICE was never able to pick up Zamarripa-Castaneda for possible deportation.

Sheriff Patrick Firman said his release goes against his department's long-standing policy.

"We knew how important it was, our staff knew how important it was," Firman said during a sit-down interview with 9NEWS. "We had the right policies and procedures in place. I was shocked to hear this happened."

For days, federal immigrations officials had wanted the city of Denver to notify them if Zamarripa-Castaneda was getting ready to post bond making himself eligible to leave the downtown jail, due to an Immigrations and Customs Enforcement hold on him.

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As we know now, that didn't happen.

"If you can't be trusted when you know the system and the media are watching you, then how do we place trust in that system when we are not watching you?" Firman said. "You know, we are going to be open we are going to be transparent. We are going to do a thorough investigation on that and we are going to be transparent with that too. I've already told you we made a mistake."

The Denver Sheriff Department has ordered an immediate internal review to try to figure out what happened.

Zamarripa-Castaneda is expected in court in April for a preliminary hearing.