"The only thing we're sure of is (it was) a dust explosion, but we have right now an undetermined ignition point," Brown said.
No flames were visible at the scene, where people began gathering within minutes of the explosion.
Kandy Brown, who stood with a cluster of people on Frederick Street near the Kenley Village shopping center, said she did not see what happened.
"No, I just heard a bang, that's all I heard," said Brown, who lives near the plant.
Donald Barton, wastewater operations manager for the city, said Synagro leases the building from the city under a contract to turn sludge into fertilizer.
Synagro was operating under a five-year renewal contract, after finishing out a 15-year agreement, Barton said. The contract is for about $79,000 a month, he said.
"They actually have a real good safety record company-wide," Barton said.
According to its Web site, Houston-based Synagro Technologies Inc. offers services that include composting, incineration, and drying and pelletizing, the functions it performs for the city.
Synagro serves more than 700 municipal and industrial facilities in 40 states, and its Web site says safety is its first priority.
Four Synagro employees were in the building at the time of the explosion, Spiker said.
Two City of Hagerstown employees also were working, Barton said.
Barton said the city will have to find an alternative way to handle its sludge, which Synagro turned into fertilizer pellets.
"They used to sell it, but the market for sales now has kind of dried up," Barton said.
Spiker said Synagro and city officials will meet to discuss who will have responsibility to pay for equipment and building repairs, but he said he could not provide a damage estimate.
The dispatcher said all of Hagerstown Fire Department's units, as well as crews from Funkstown and Halfway responded to the scene. Rescue units clogged Frederick Street, which temporarily was blocked from Wilson Boulevard to Kenly Avenue.
A sign in front of the wastewater plant said City of Hagerstown employees have logged 270 workdays without a lost-time injury.