DENVER (AP) — On Monday, the U.S. Department of Transportation issued a stern warning to Colorado, threatening to withhold $24 million in federal funding due to the state’s sluggish response to a significant breach of federal commercial driver’s license regulations.

Transportation Secretary Sean Duffy highlighted findings from a nationwide audit conducted in October. The audit revealed that approximately 22% of commercial driver’s licenses issued in Colorado to immigrants were granted illegally, predominantly to Mexican nationals—a clear violation of federal law.

Duffy criticized Colorado for “dragging its feet” in the necessary revocation of these improperly issued licenses. He noted that the state has yet to conduct a comprehensive audit, provide a full list of affected drivers, or revoke the invalid licenses, despite being alerted to the compliance issue.

Emphasizing the importance of adhering to federal regulations, Duffy stated that Colorado cannot selectively enforce rules, especially when public safety is at stake.

“Each day that passes poses a risk, as unqualified and unvetted foreign truck drivers continue to endanger the safety of you and your family,” Duffy remarked in his statement.

In addition to Duffy’s ultimatum, he also noted that the Transportation Department has the authority to decertify Colorado’s entire CDL program if the state doesn’t act swiftly.

Colorado Gov. Jared Polis’ office did not immediately respond to emailed questions about the federal government’s allegations or the potential public safety concerns.

Polis, a two-term Democrat, has been at odds with the Trump administration in recent months. Tensions flared as recently as this past weekend when the Trump administration denied disaster declaration requests following wildfires and flooding earlier this year. Last week, Polis also criticized the administration’s plans to dismantle a federal climate research lab located in the state.

Colorado’s Division of Motor Vehicles has paused the issuance and renewal of term-limited non-domiciled CDLs and commercial learner’s permits indefinitely, The Denver Post reported last week. That pause is expected to remain in place pending an audit of all such licenses to ensure compliance with federal regulations.

Duffy has threatened federal highway funding for other states, too, in his effort to make sure truck drivers and bus drivers are qualified to either haul passengers or 80,000 pounds (36,000 kilograms) of cargo down the highway.

New York was the fourth state run by a Democratic governor to be called out publicly earlier this month. Questions also have been raised about similar practices in California, Pennsylvania and Minnesota.

Letters also have gone out from the agency to Republican-run Texas and South Dakota.

Duffy launched the review this summer, but it became more prominent after officials reported that a truck driver who was not authorized to be in the U.S. made an illegal U-turn and caused a crash in Florida that killed three people in August.

The rules on these licenses the Transportation Department is enforcing have been in place for years. According to the federal audit, it was discovered that in some instances states may not have even checked a driver’s immigration status before issuing a license.

Since Duffy began pressing the issue in California, that state has revoked some 21,000 commercial driver’s licenses that were issued improperly.