The chief federal prosecutor in Boston said the initial investigation into two pipe bombs that were mailed to a scientist in 1993 was "appropriate and thorough." The case has never been solved.
Amy Bishop was charged in February with the University of Alabama-Huntsville shootings and in June with her brother's 1986 killing in Massachusetts. She and her husband were questioned about the 1993 pipe bombs, but were never charged.
Dr. Paul Rosenberg had received the bombs shortly after Bishop left her job as a researcher at Children's Hospital in Boston, partly due to a poor review by Rosenberg. They did not detonate.
U.S. Attorney Carmen Ortiz decided to review the investigation after Bishop was charged with the Alabama shootings.
"This office and the investigative agencies assigned to the matter engaged in extensive efforts to determine the source of the incendiary device, but, despite those efforts, were unable to gather sufficient evidence to bring charges," Ortiz said in a statement Thursday. "As a result, the matter was closed."
"This office does not intend to reopen the matter and we will have no further comment," the statement read.
During the 1993 investigation, 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 said "he wanted to get back at victim Dr. Rosenberg and that he wanted to shoot him, bomb him, stab him or strangle Rosenberg."
Her husband, James Anderson, has said he and his wife were among several innocent people interviewed by authorities, and that they were not suspects.
After Bishop was charged in the University of Alabama-Huntsville shooting rampage, Ortiz said she wanted to review the 1993 investigation to "confirm that all appropriate steps were taken." First Assistant U.S. Attorney Jack Pirozzolo conducted the review.
Bishop 45, a former biology professor at the school, is charged with capital murder and attempted murder in the Alabama shootings, which killed three colleagues and wounded three others.
After the killings, stories of violence in Bishop's past surfaced.
In June, a grand jury indicted Bishop for murder in her brother's 1986 killing after a judge held an inquest into the death. The death of her brother, Seth, was originally ruled an accident after Bishop told police she accidentally shot him in the family's Braintree home while trying to unload her father's shotgun.