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U.S. Postal Inspector Frank Schissler, a spokesman for the Washington division of the inspection service, said Friday that investigators were examining postmarks and other exterior markings on the packages in an attempt to trace their origin.

The postal service also will examine its internal tracking data, Mr. Schissler said. Packages are tracked once they enter mail processing plants. But the packages did not have individual tracking numbers because they were sent by first-class mail, not registered mail or express mail, he said. Schissler also said that DNA analysis was likely.

A mail carrier was delivering and picking up mail Friday morning at the Jeffrey Building in Annapolis, where the first package was opened, and other state office buildings. Workers met the mail carrier’s truck on the street near the governor’s mansion to exchange outgoing mail for incoming mail. They declined to comment.

Rep. C.A. Dutch Ruppersberger, Maryland Democrat, said a return address on one of the packages turned out to be a Washington parking garage. Mr. Ruppersberger, a member of the House Intelligence Committee who was briefed on the mailings, said there were no apparent links to terrorist organizations.

“I believe this is what we call in intelligence a lone wolf situation, involving an individual who for whatever reason was upset with state government,” Mr. Ruppersberger said.

The Department of Transportation is subjecting mail to additional scrutiny, but otherwise operations were back to normal Friday, spokesman Jack Cahalan said.

After the administrative assistant opened the package Thursday, she dropped it on the floor and someone pulled a fire alarm. Mr. Cahalan, who was on the fourth floor but did not see the package being opened, said he initially thought the alarm was a drill. About 250 people work in the four-story building, and the evacuation was orderly, he said.

“I’ve participated in more fire drills here than I ever did in elementary school,” he said. “Everybody knows the drill; everybody knows what to do.”

The FBI’s joint terrorism task force was assisting in the investigation. A U.S. Homeland Security Department official said the department was aware of what happened and was monitoring.

Postal inspectors have identified 13 dangerous devices sent through the mail since 2005, and only one person was injured, according to the U.S. Postal Service. Inspectors made arrests in eight of those cases, said Mr. Schissler, who noted that the packages sent Thursday would not be classified as dangerous because they did not contain bombs.

In 2001, as the nation was still reeling from the 9/11 attacks, letters containing anthrax were sent to lawmakers and news organizations. Postal facilities, U.S. Capitol buildings and private offices were shut for inspection and cleaning. The anthrax spores killed five people and sickened 17.

Associated Press Writers Alex Dominguez and Kasey Jones in Baltimore, Sarah Brumfield and Jessica Gresko in Annapolis, Brian Witte and Norm Gomlak in Atlanta, and Eileen Sullivan and Alicia Caldwell in Washington contributed to this report.