"We are proceeding as though it is a hazard, because we don't know if it is or not," Klaver said. "It could be anything from a hazardous material to flour with yeast in it."
Tests may be completed within the next 24 to 48 hours, Klaver said.
The Associated Press reported that Thursday evening that Independence National Historical Park spokeswoman Jane Cowley said multiple tests determined the powder was flour.
The small quantity of powder, which was inside a blue balloon that was left on a mat at the entrance to the Liberty Bell Pavilion, was spotted around 2:30 p.m. by a private security guard. The guard and two others picked it up and placed it in a plastic bag, then called police.
The discovery led authorities to quickly close 6th Street between Arch and Chestnut Streets. Police, the Philadelphia Police Homeland Security unit, the Philadelphia fire department, and the FBI Hazardous Material unit descended on the scene.
Traffic backed up on the streets surrounding the historic Old City area, and curious tourists stopped and stared. Reporters and cameras clustered on the corners as television news helicopters circled overhead.
Authorities combed the area for further traces of the substance and set up a decontamination booth, although it appeared they did not have to use it. Neither the guards or a firefighter who came into contact with the powder appeared to have suffered any ill effects, Klaver said, and no one was hospitalized.
The area was reopened by 7 p.m.
Field tests determined that the powder is a biological substance that contains proteins, Klaver said. The powder was shipped to a Pennsylvania lab Thursday for further testing.
Depending on the results of those tests, authorities might review security footage from surveillance cameras at the scene, Klaver said.
Elizabeth Battle, who was visiting Thursday from New York, had the misfortune of parking on 6th Street while she went on a Duck Boat tour. When she returned, she found her car stuck in the middle of the scene.
"It's been two and a half, three hours," she said as she waited for police to allow her to retrieve her car. "But it is a threat. I understand."
Contact staff writer Allison Steele at 215-854-2641 or firstname.lastname@example.org.