An unemployed factory worker was arrested last night on suspicion of setting off a small pipe bomb at the Twain Harte Post Office on Thursday night.

Dana Jesiah Workman, 23, who for a week has lived in a house on the 300 block of Fairview Lane in Sonora, remained this morning in the Tuolumne County Jail in lieu of $50,000 bail. Workman previously lived in Eureka, said Tuolumne County Sheriff's spokesman Lt. John Steely this morning.

Steely said investigators developed leads right after the explosion that made Workman the prime suspect.

About 9:30 last night, deputies stopped a vehicle on Tuolumne Road near West Side in which Workman was a passenger, Steely said, and he was brought in for questioning. Based on his comments, investigators obtained a search warrant for the Fairview Lane house and found explosive devices and illegal fireworks, Steely said.

Information on what led investigators to Workman was not released.

The pipe bomb was set off about midnight Thursday on the right side of the Twain Harte Post Office, next to the parking lot. The building was not damaged and no one was hurt, but the explosion sent shrapnel all over the lot and into Fuller Road.

A sheriff's deputy was in Twain Harte, about a block away, when he heard the loud blast and began searching the area.

With the help of a CDF fire crew stationed in Twain Harte, the deputy discovered where the blast happened and quickly cordoned off the post office and parking lot. Witnesses said they saw a car with four people in the area just before the bomb went off.

Friday morning, investigators from the state office of the FBI, Alcohol Firearm and Tobacco, CDF Bomb Squad along with Tuolumne County Sheriff and the Calaveras County Hazardous Material Bomb squad meticulously combed the parking lot for evidence.

The post office was closed Friday, but reopened Saturday morning.