By Nate Raymond

BOSTON (Reuters) -A Russian-born scientist at Harvard University who was detained by U.S. immigration authorities in February after returning from a trip to France was granted bail on Thursday and released from custody while she awaits trial on a criminal charge of smuggling frog embryos into the United States.

A federal magistrate judge in Boston released Kseniia Petrova, 30, two weeks after a different judge in Vermont granted her bail in her ongoing immigration case after concluding it appeared she was unlawfully detained.

The May 28 decision by U.S. District Judge Christina Reiss concerned only Petrova's immigration case, though, and left unresolved the question of whether she would be granted bail in a subsequent criminal case prosecutors filed two weeks earlier.

U.S. Magistrate Judge Judith Dein did just that during a hearing in Boston, after prosecutors dropped an earlier request to detain her, on the condition that she remain in New England while the criminal case is pending.

"I just want to thank everybody for supporting me," Petrova in a video told reporters gathered outside the courthouse.

Petrova's detention came as Republican President Donald Trump's administration ramps up deportations as part of its wide-ranging efforts to fulfill the president's hardline immigration agenda.

Her supporters said she was taken into custody as part of the Trump administration's practice of targeting international students and academics for visa revocations and detention, leading to a series of court challenges.

Petrova, who works at Harvard Medical School, was detained at Logan International Airport in Boston on February 16 on her return from a trip to France.

Prosecutors said U.S. Customs and Border Protection agents stopped Petrova after her checked duffle bag was flagged for inspection, revealing the frog embryos.

Petrova has said her boss asked her to bring back frog embryo samples for experiments. But prosecutors said the embryos constituted biological material that should have been declared to customs officials and required a permit.

Petrova's visa was then canceled and immigration officials took her into custody with the intent of deporting her back to Russia, a prospect Petrova has said she feared after protesting Russia's war in Ukraine.

She was held for months in an immigration detention facility in Louisiana and was only charged and moved into criminal custody shortly after Reiss heard arguments in a lawsuit Petrova filed challenging her detention.

(Reporting by Nate Raymond in Boston; Editing by Bill Berkrot)