BANGOR, Maine — A former postal worker pleaded guilty Thursday in U.S. District Court to illegally opening a package while she was employed at the sorting facility in Hampden.

Heather Buck, 40, of Brewer is scheduled to be sentenced Oct. 21 for obstruction of mail.

Buck was the second former worker at the Hampden facility in a month to plead guilty to committing a crime while on the job. Christopher McBride, 39, of Bangor, pleaded guilty Sept. 16 to theft of mail by a U.S. postal employee. His sentencing date has not been set.

By pleading guilty to the federal misdemeanor, Buck admitted that on Sept. 12, 2010, while employed as a flat sorting machine operator, she opened a package that contained a book, thumbed through it, then repackaged it.

The title of the book was not in court documents.

Buck’s attorney, Lawrence Lunn of Bangor, told U.S. Magistrate Judge Margaret Kravchuk on Thursday that his client found a partially damaged package and opened it looking for a packing slip, as she had been instructed to do in such cases by her supervisors.

Supervisory Assistant U.S. Attorney Todd Lowell, who is prosecuting both cases, told Kravchuk that Buck’s case was linked to McBride’s. The investigation that resulted in Buck and McBride being charged began in September 2010 after the U.S. Postal Inspection Service received information that veterans living in eastern Maine had not received prescription drugs that had been mailed to them, according to court documents. Between June 11, 2010, and Sept. 3, 2010, 15 parcels containing narcotics were reported missing by the Veterans Affairs Office of the Inspector General.

Surveillance cameras were set up at the Eastern Maine Processing and Distribution Center in Hampden, where Buck and McBride worked. Buck was caught on video opening the package containing the book on Sept. 12, 2010, according to information in the prosecution version of events to which she pleaded guilty.

McBride was recorded on Sept. 15 taking “two white parcels from the mail stream and placing them in his backpack,” according to the prosecution of events to which he pleaded guilty. Further surveillance in September, October and November captured McBride taking several additional packages containing prescription drugs. McBride admitted to stealing more than 1,000 pills from the mail.

Buck and McBride remain free on personal recognizance bail pending sentencing.

She faces up to six months in prison and a fine of up to $5,000 on the federal misdemeanor charge. He faces up to five years in prison and a fine of up to $250,000.