US denies extradition request for Anne Sacoolas, diplomat's wife who allegedly hit, killed British teen

Anne Sacoolas allegedly hit and killed Harry Dunn with her car.

The U.S. has denied an extradition request for Anne Sacoolas, the wife of an American diplomat who police say fatally struck a British teenager with her car as he was walking on the side of the road last August.

Sacoolas was formally charged with murder in December for allegedly hitting 19-year-old Harry Dunn in Northamptonshire, in central England. Sacoolas was charged by the Crown Protection Service with causing death by dangerous driving.

Radd Sieger tweeted, "[MP Andrea Leadsom] @andrealeadsom tells me 2night that @SecPompeo has declined the extradition request for #annesacoolas. The first declination in the history of the treaty. #HarryDunn's parents expected nothing less of this immoral lawless admin.They don't know what they have let themselves in 4!"

Sacoolas initially cooperated with investigators, but then fled to the United States claiming diplomatic immunity, which sparked a diplomatic rift between the U.S. and the U.K.

A State Department spokesperson confirmed to ABC News in a statement that they denied the U.K. extradition request -- again saying if they approved it and waived the immunity they maintain she had during the accident and while she remained in the U.K., it "would set an extraordinarily troubling precedent."\

After word of the U.S. decision, the British government condemned the decision.

"We are disappointed in this decision which appears to be a denial of justice," the British government said in a statement. "We are urgently considering our options."

The U.S. secretary of state has the final word on whether to honor a request from another country, in the U.K.'s case through the Home Office.

Sieger followed up with another tweet saying the extradition fight was not over.

"Don't you worry #HarryDunn supporters. Taking this in our stride. #annesacoolas is coming back," he wrote. "You wont stand for it. We won't. British Govt won't. Next steps to be discussed and agreed, and they will be ferocious."

The U.S. government issued a statement in the wake of the extradition request earlier this month making it clear the State Department was unlikely to honor the request.

"The United States has a strong law enforcement relationship with the UK and, in particular, a strong track record of close cooperation on extradition matters," according to a spokesperson from the State Department. "However, under the circumstances of this case, we strongly believe that an extradition request would be highly inappropriate."

ABC News' Guy Davies, Conor Finnegan and Ella Torres contributed to this report.