An improvised explosive device rocked the Jackson County District Attorney’s Office early Wednesday, shattering windows and sparking a fire that damaged the downtown building.

No one was injured.

A fuse meant to detonate a 7-gallon propane tank was destroyed in the explosion, said Medford police Lt. Mike Budreau. He said the propane tank partially detonated.

The city's police and fire departments initially responded to the scene, and have since been joined by explosives experts from the Oregon State Police and the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives. Additional agents from the ATF's Virginia office are en route.

Investigators have no suspects, said Medford Chief Tim George. He said the person behind the explosion was “trying to send a message.”

“The why is what we are chasing right now,” George told The Oregonian. “The why will get us to who.”

Police and fire officials initially responded to a report of an explosion at 4:30 a.m. after multiple witnesses, including police officers, heard a loud sound up to a mile from the district attorney's office.

"They said it sounded like a thunder clap," Budreau said.

As police headed to the building, one encountered a man who ran from him. Budreau said it’s not clear if that incident, which took place a quarter-mile away, is related to the explosion.

“The officer did give chase and did not locate him,” he said.

The propane tank sent a “huge blow torch” up the front of the building, Budreau said. The tank is larger than one used on an outdoor grill, he said, and is more commonly used in recreational vehicles.

The device was placed to next to the entrance, he said.

“You could obviously tell there had been an explosion at the front,” he said. “The good news is that propane tank did not fully detonate.”

The single-story brick building, at 715 West 10th St., houses the district attorney's office, which has about 30 employees. The office is overseen by District Attorney Beth Heckert, the first woman to hold the post in the county. She was elected in May 2012.

The building is part of the campus of criminal justice buildings in the area. Officials worried about additional explosives in the area so firefighters let the flames go out on their own, Budreau said.

Investigators are combing the scene for any trace of the explosive, he said.

“We are going to be real slow and by the numbers,” Budreau said. “Everything we have out there, every piece of shrapnel to the (propane) can itself is evidence and the best way to find out who did this.”

William Fowler, a business lawyer with offices next to the district attorney’s office, couldn’t get in his front door when he arrived at work about 8 a.m.

“The whole street is taped off,” he said. “There are a whole bunch of police cars out there.”

Authorities this morning swept neighboring government buildings for additional explosives but found none.

An officer let Fowler go into his offices through the back door for a 9 a.m. phone hearing.

The nearby Justice Center remains blocked off, employees said.

Administrative assistant Terri Galedrige arrived at the nearby Justice Center about 7 a.m. for work, as police were taping off the street. The Justice Center was not affected but police swept the four-story building before allowing Galedrige and four other staffers go to work. She said they had to call jurors and other employees to alert them not to show up.

“It took us about an hour,” Galedrige said.

Galedrige said this isn’t the first such threat against the court system but the first explosion.

“We’ve had bomb threats but never one that actually went off,” she said. “I’m just glad no one got hurt.”

Still, it’s worrisome, she said.

“You never know if there’s something that they haven’t found,” she said.

The building will reopen at 1 p.m., Galedrige said. All hearings and trials scheduled for Wednesday morning have been postponed, either to Wednesday afternoon or Thursday, she said.

-- Noelle Crombie, Lynne Terry and Bryan Denson