The deadline was issued March 24 ... and it has been ticking down ever since.

Now, with exactly 15 days to go, prominent environmental regulators have doubts that Volkswagen will be able to hit its April 21 deadline to come up with an agreed-upon proposal for how it's going to bring nearly 600,000 compromised diesel vehicles into compliance with U.S. emissions standards.

According to Reuters, Environmental Protection Agency (EPA) administrator Gina McCarthy told reporters Tuesday that although VW and regulators are engaged in "really robust" talks, she's uncertain if they can come to terms on an agreement for a possible fix by April 21. Within that, she didn't divulge about whether regulators would possibly accept the embattled automaker buying back manipulated vehicles or coming up with a way to partially fix them.

Why the doubt? Well, because VW may still not have a grip on how to fix its ongoing emissions fix in a way to appease the EPA and California Air Resources Board (CARB), with a full fix possibly being out of the question altogether.

Last month, a California regulator revealed that the state could allow partial fixes on compromised diesel vehicles, although McCarthy seemed to cast slight doubt on that possible remedy, too.

"I wouldn't take any single signal as the direction of those negotiations or where any final agreement - if we reach one - will end up," McCarthy said, as reported by Reuters.

Yet, a VW spokesperson vowed to Reuters that the company is working hard in talks with the EPA and CARB to come up with a possible fix within the next 15 days.

This dicey situation comes after optimism when U.S. District Court Judge Charles Breyer originally issued the automaker the deadline on March 24.

"I would hope by the 21st that as many astounding issues as possible will be wrapped up," Breyer said at the time, according to CBS.

CBS also reported that Breyer said ex-FBI director Robert Mueller informed him that VW, the EPA, CARB and lawyers representing class-action lawsuits for affected vehicle owners have made progress in their talks and that a resolution is expected.

If VW fails to strike a deal with U.S. regulators, Breyer could put the company on trial as soon as this summer.

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