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Maryland officials ordered mailrooms at all state agencies closed indefinitely Thursday, while at the University of Maryland, College Park police quickly shut down the campus' central mailroom and sent out alerts urging that no mail be opened.
The reactions came swiftly after two incendiary devices ignited in government offices in Annapolis and Hanover, from authorities likely mindful of past deadly letter bomb incidents as well as the 2001 anthrax attacks that killed five people and sickened 17.
Matt Gallagher, chief of staff to Gov. Martin O'Malley, notified Cabinet secretaries in a terse e-mail. "Please direct your administrative staff to cease mail delivery and processing," he wrote, "and close and secure all mailrooms until further notice and direction."
It is unclear how long the order will remain in effect and how it might impact the functioning of state agencies. O'Malley spokesman Rick Abbruzzese said later that he did not expect the agency mailrooms to be affected for very long. "Each mailroom will be investigated and cleared," he said.
Recent incidents have served as reminders of the lethal potential of mail bombs.
Two parcel bombs exploded last month inside the Swiss and Chilean embassies in Rome, wounding two men. An anarchist organization claimed responsibility for at least one of the blasts, according to news reports.
And in October, authorities in Britain and the United Arab Emirates intercepted packages sent from Yemen containing powerful bombs. American officials have said the bombs appeared to have been designed to blow up airliners on their way to the United States, adding that they bore the hallmarks of a fugitive Saudi bomb-maker who has targeted the United States.
In the United States, officials note that letters have been used as weapons practically since the advent of mail delivery.
"For over 250 years, it's safe to say, we've been investigating things like this," said Inspector Michael Romano of the U.S. Postal Inspection Service.
Between 2005 and 2009, he said, inspectors investigated 13 confirmed mail bomb incidents nationwide. None of those caused any injuries because they didn't explode as intended.
Perhaps the most notorious mail bomber was Theodore Kaczynski, the "Unabomber." He killed three and injured 29 in a series of 16 mail bombings across the country between 1978 and 1995.
In 1989, U.S. Circuit Judge Robert S. Vance Sr. died when he opened a package bomb at his home near Birmingham, Ala. Walter Leroy Moody was convicted in the mail bomb killings of Vance and Georgia lawyer Robert Robinson. Moody remains on death row.
In March, at least five packages containing bullets, harmless white powder and threatening letters were sent through the U.S. Postal Service to judges at the Baltimore Circuit Courthouse, though no one was hurt in those incidents.
Romano said postal inspectors in 2009 investigated 88 cases of powders and other suspicious substances being sent through the mail. The investigations resulted in a dozen arrests.
In the anthrax attacks of 2001, letters containing the deadly spores killed five people and sickened 17 others. The FBI declared the investigation closed after Bruce E. Ivins committed suicide in 2008. Ivins, who worked at the U.S. Army Medical Research Institute of Infectious Diseases, was considered the sole suspect in the case, according to the FBI.
At College Park, university police sent a message Friday morning telling everyone on campus that it was safe to handle mail again. The message, which went out at 9:16 a.m., said that police had screened all recently arrived mail and found no indication of incendiary devices.
Campus police prepared Thursday for a comprehensive search of packages in the mailroom, with a particular focus on any parcels that might resemble those that ignited in Anne Arundel County. The campus is relatively empty, with most students on winter break until January 24.
The decision to close the mailroom Thursday was precautionary and not related to any specific threats to the university, Leonard said. "We don't have any kind of specific information, but we are a state agency," he said.
The state university system allowed its campuses to respond on a case-by-case basis.
At the University of Maryland, Baltimore, a campus alert at 4:23 p.m. said no suspicious packages were found in a preliminary search of the mailroom. But the mailroom remained closed, and employees were sent home early.
Other campuses, such as Towson University, responded with information alerts rather than closing mailrooms or warning against opening mail.
Baltimore Sun reporters Childs Walker, Annie Linskey and Julie Bykowicz contributed to this article.