A backlog of diesel cars waiting to be tested by federal regulators has delayed several models from going on sale in the U.S. including many from BMW and possibly others from different manufacturers.

And the backlog only compounds the problems diesel-powered passenger cars and SUVs face in the U.S., alongside low gas prices and plummeting public opinion.

Automotive data firm Wards Auto reported that sales of diesel-powered cars nearly ground to a halt at the beginning of 2016, compared with the same time period in 2015. In January 2016, just 222 diesel-powered cars were sold, compared to nearly 4,500 in the same month a year before. 

Last week, BMW confirmed that it wouldn't begin production of most of its diesel models until federal testers completed review of its diesel cars, which could mean that even if the cars are certified it would be months to get the slow-selling models to market—if they come at all.

"The product certification of all MY2017 BMW diesel models (328d Sedan, 328d Sports Wagon, X3 xDrive28d, X5 xDrive35d) has been delayed due to testing logistics. As a result, production of these models will commence once homologation has been finalized," a spokesman for BMW said in a statement.

Officials from the Environmental Protection Agency didn't comment on the testing delays after repeated requests.

VW announced this month that many of its diesel models wouldn't return to the U.S. after that company's publicand still ongoing—cheating scandal.

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“We are not stopping diesel. Wherever diesel makes sense as a package to the car, we’ll continue,” Hinrich Woebcken told Automotive News. “But in reality, we have to accept that the high percentage of diesels that we had before will not come back again.”

Earlier this year, Mercedes-Benz officials confirmed that their entry-level GLS-Class SUV was undergoing further testing by the EPA. That model hasn't yet arrived in the U.S. and has disappeared from Mercedes' online configuration tool for potential customers.

A spokesman for Mercedes confirmed this month that the model would be available in the U.S., but didn't say when it could go on sale.

Last year, sales of the diesel-powered Chevrolet Colorado and GMC Canyon were initially held up due to increased testing by the EPA. Those diesel trucks weren't delivered to many consumers until late December or early January, nearly two months after receiving their EPA certifications.