It is now significantly harder for non-U.S. citizens to obtain a commercial driver's license under new rules the Transportation Department imposed Friday.

Transportation Secretary Sean Duffy said an audit that was done after a deadly crash in Florida last month that was caused by a truck driver attempting an illegal U-turn showed that the previous rules weren't strict enough and a number of states weren't following them consistently. Duffy has said that truck driver should have never received a commercial license because of his immigration status.

That review found that commercial driver's licenses were improperly issued in California, Colorado, Pennsylvania, South Dakota, Texas and Washington. But Duffy said the problems were so egregious in California that he is threatening to pull $160 million in federal funding.

Here's what is changing and what led to the changes:

The nationwide audit was kicked off after Harjinder Singh, an India-born truck driver, made an illegal turn on Florida's Turnpike, according to the Florida Highway Patrol. A minivan behind it wasn't able to avoid the truck's trailer and crashed into it. The driver and two passengers were killed, while Singh and a passenger in his truck were not injured.

Singh lived in California and was originally issued a commercial driver's license in Washington before California issued him one. He has been charged with three state counts of vehicular homicide and immigration violations.

Singh's attorney, Natalie Knight-Tai, declined to comment immediately on the case.

Officials have also pointed to two other fatal crashes this year they say were caused by immigrant truck drivers who never should have received licenses. A driver in Texas failed to brake and crashed into a line of cars, resulting in a 17-car pileup that killed five people. And in Alabama, a driver struck four vehicles stopped at a red light, killing two people.

The Florida crash and subsequent investigations have fueled arguments between the governors of Florida and California while eliciting outrage from President Donald Trump and Transportation Secretary Sean Duffy.

Duffy said the audit found instances in which California, Colorado, Pennsylvania, South Dakota, Texas and Washington issued licenses improperly, saying the rules need to be stricter.

But he has homed in on California, where he said investigators found that one in four of the 145 commercial drivers licenses for noncitizens issued since June that they reviewed should have never been issued under the current rules. California now has 30 days to audit its program and present a plan for complying or risk losing its funding.

California has pushed back on these claims, with a spokesperson for Gov. Gavin Newsom saying Duffy doesn't understand federal law and claiming that commercial driver's license holders from California have a significantly lower rate of crashes than both the national average and the Texas average, which is the only state with more licensed commercial drivers.

Duffy had previously threatened to strip some federal funding from California, Washington and New Mexico after he said they didn't enforce English proficiency requirements for truckers that went into effect this summer.

California has been a battleground for the Trump administration, with the president and Newsom regularly taking to social media to air their grievances about the other.

The new rules only allow noncitizen drivers who have three specific visas to qualify. And states will be required to verify their immigration status. Only drivers who hold either an H-2a, H-2b or E-2 visa will qualify. H-2B is for temporary nonagricultural workers, while H-2a is for agricultural workers. E-2 is for people who make substantial investments in a U.S. business

The licenses will only be valid for up to one year unless the applicant's visa expires sooner than that.

Officials said that out of 200,000 noncitizens who currently have commercial licenses, only about 10,000 of them would qualify under the new rules. But the rules aren't retroactive, so those drivers will be able to keep their licenses at least until they come up for renewal.

All states must pause issuing commercial driver's licenses to noncitizens until they can comply with the new rules.

The 200,000 noncitizen truck drivers on the road represent about 5% of all the commercial drivers. Duffy said he thinks there are enough American drivers to deliver all the loads.

Removing noncitizen drivers from the industry could force trucking companies to increase wages for entry-level operators and draw more job seekers, said Jonathan Marques, founder of the Driving Academy in Linden, New Jersey.

Marques said that could even make the industry more attractive.