New York prosecutors have dropped a harassment case against a sister of the Boston Marathon bombers, saying allegations she threatened to "put a bomb" on a perceived romantic rival couldn't be proved.

The Manhattan district attorney's office said Monday that it had "significant" questions about the credibility of the woman who accused Ailina Tsarnaeva (tsahr-neye-EH'-vuh).

Tsarnaeva's lawyer hasn't immediately returned a call. Authorities haven't publicly identified Tsarnaeva's accuser.

Prosecutors had said Tsarnaeva had threatened a woman by saying she knew "people that can put a bomb where you live."

The death penalty phase of the federal trial of Tsarnaeva's brother Dzhokhar Tsarnaev (joh-HAHR' tsahr-NEYE'-ehv) is unfolding in Boston. He was convicted last month in the 2013 bombing.

Their brother Tamerlan was killed during the subsequent manhunt.