U.S. Immigration and Customs Enforcement (ICE) issued a subpoena to Denver law enforcement Monday for information on four undocumented immigrants who are slated for deportation after arrests for violent crimes.

The action comes as federal officials continue to criticize so-called sanctuary cities, which don't cooperate with immigration officials by refusing to hold jail inmates past their release date in order for ICE to collect them.

“We are reviewing the administrative subpoenas from ICE, which were not issued by a court of law,” said Theresa Marchetta, the director of strategic communications with Denver Mayor Michael Hancock's office. “We want to be very clear that our immigration ordinance fully complies with federal law.”

The four men -- three from Mexico and one Honduran -- were previously deported, ICE said. Three of them have been released from a Denver jail and one is still in custody. They were arrested for violent offenses, such as sexual assault of a child and child abuse.

The city has 14 days to respond with information in the three cases and three days to respond in the fourth. The agency said if the city doesn't respond, it will go to a federal judge to force compliance.

“Since we have no cooperation at the Denver justice center, we are modifying our tactics to produce information,” said Henry Lucero, deputy executive associate director for ICE’s Enforcement and Removal Operations.

He called the subpoena a last resort, saying he doesn't want ICE to be in the business of subpoenaing other law enforcement agencies.

“This is a drastic change,” he said of the subpoenas. “And one ICE is forced to do, and puts other agencies on notice that we don’t want this to happen. We want to protect the public.”

Messages to ICE and Denver officials from Fox News were not immediately returned.

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Former Attorney General Jeff Sessions sent letters to 29 cities, metro areas and states considered to have adopted sanctuary policies. The Trump administration has blasted such cities and threatened to withhold law enforcement grants.

Critics say sanctuary cities protect violent criminals while supporters argue they create environments where undocumented immigrants can feel safe and can report crimes without fear of deportation.

The Associated Press contributed to this report.