"Someone poured gasoline ... then lit it, then 'boom,'" Assistant Washington County State's Attorney Theresa Adams said in her opening statement.
Adams said Mitchell was seen running from the building after the explosion and driving away in a car he rented three days earlier.
Six days before the explosion and fire, Mitchell took out a $50,000 insurance policy, Adams said.
"This is a very serious case ... jeopardizing many lives," she said.
Mitchell's attorney, Wiley Rutledge, urged the jury to remember two words: "white van."
If investigators in the Washington, D.C., sniper case only looked for a white van - a mistaken early report of the sniper vehicle - "those idiots would still be running around," Rutledge said.
Two witnesses who identified Mitchell had been in an altercation with him, Rutledge said.
Also, he said, a caller to 911 thought the getaway vehicle was a four-door car - which didn't match Mitchell's car.
"Please keep an open mind," Rutledge said.
Mitchell is charged with four counts each of first-degree arson and second-degree arson; two counts of reckless endangerment; and single counts of first-degree malicious burning and insurance fraud.
There are four apartments at 38-40 Randolph Ave. Mitchell lived in the upstairs apartment at 40 Randolph Ave.
Hagerstown Fire Department Capt. Justin Mayhue testified Monday that the building was blazing when he got there about a minute after a dispatcher put out a call.
"It was unusual to have that volume of fire that quick," he said.
Mayhue said the severity of the fire blew a wall from 40 Randolph Ave. onto 42 Randolph Ave., setting it on fire, too.
The jury also heard a tape of several 911 calls reporting the fire.
"A guy threw a bomb into a house and took off," one caller said. Another reported seeing "the guy who did it" run from the house.
Two people said they saw the arsonist's car. "It looked like a four-door," said one person - the caller to whom Rutledge referred.
Washington County Circuit Judge John H. McDowell told jurors that the trial could last five days.