"The sound you heard was our device working properly," Meminger said.
After the package was opened, Meminger said, it was turned over to postal investigators.
Bruce Rice, the acting officer in charge of the post office, said an employee discovered the 8-by-10 foot flat package in outgoing mail at about 8:15 a.m. He said he called postal authorities who contacted the fire marshal's office.
Tammara Illig, a supervisor of customer services, said the package drew attention because it had not addressee and it was making hissing and popping noises.
Rice said the contents were destroyed, but did not specify what they were. After determining the package did not contain explosives, Meminger said it was turned over to postal investigators.
U.S. Postal Inspector Paul Trimbur, in Baltimore, said postal employees are trained to look for warning signs, including incorrect address information, missing return address information or too much postage. People mailing bombs often put too many stamps on a package in order to avoid having an employee weigh it, he said.
Illig said officials usually do not call bomb technicians unless they observe something dramatic, such as noises.
Trimbur said noises are almost always a toy or other harmless item.
"No expert bomber is going to devise a bomb with ticking," he said.
Trimbur said reports of suspicious packages at post offices are rare and are taken seriously. He said a 2-year-old law that limits the weight of packages that can be mailed in drop boxes has helped. A package that weighs more than 16 ounces must now be mailed at the post office.
Robert Gingell, acting manager of post office operations for the Baltimore district, said the first priority is safety.
Rice said the scare put drivers about 90 minutes behind schedule.
Mail carrier Tonya Detrick said she was packing up the mail when employees were told to leave the building.
"We were working the mail and the supervisor said over the intercom for everyone to evacuate the building; that there was a suspicious package," she said. "It is scary, considering a bomb could take many of our lives."