The U.S. attorney's office in Boston said Wednesday that it was reviewing its investigation into an attempted mail bombing, an inquiry in which authorities questioned a professor now accused of killing three colleagues at a university.

Amy Bishop and her husband were questioned, but not charged, in the 1993 attempted bombing. Dr. Paul Rosenberg received the bomb, which did not explode, shortly after Bishop quit her job at Children's Hospital following a poor review by Rosenberg.

Bishop is accused in the Feb. 12 shooting deaths at the University of Alabama in Huntsville, where she was a professor.

In a statement, U.S. Attorney Carmen Ortiz said her office had started "a thorough review" of its information about the 1993 case "to confirm that all appropriate steps were taken in that matter, and to determine whether information related to this incident may be of assistance to other law enforcement agencies."

Ortiz said her office would not comment further until its review was finished.

Bishop 45, is charged with capital murder and attempted murder in the Alabama shooting, which wounded three other colleagues. Police have not offered a motive, but colleagues say she had complained for months about being denied the job protections of tenure.

Since the killings, stories of violence in Bishop's past have surfaced.

She killed her brother in 1986, which authorities at the time ruled an accidental shooting. The investigation has been broadly criticized in light of the Alabama case.

In the 1993 case, Rosenberg told authorities Bishop had resigned her job as a postdoctorate research fellow with him around the time he was mailed the pipe bomb. Rosenberg said that "he had been instrumental in her leaving because he had felt she could not meet the standards required for the work," according to a report by the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives.

A witness also told the ATF that Bishop's husband, James Anderson, said "he wanted to get back at victim Dr. Rosenberg and that he wanted to shoot him, bomb him, stab him or strangle Rosenberg."

Anderson has said he and his wife were among several innocent people interviewed by authorities, and that they were not suspects.