Guard officials announced that three soldiers were killed in the explosion and five others were wounded.
The dead were identified as Sgt. Edward Koehler, 47, of Lebanon; Sgt. Brian Mowery, 49, of Halifax, north of Harrisburg; and Staff Sgt. Kenneth VanGiesen, 30, of Kane, in the big woods of Northern Pennsylvania.
The Guard did not release the names of the wounded, but said four of the five would be able to return to duty shortly.
The fifth had been flown to a U.S. military hospital in Germany and was expected to live.
The deaths brought to 39 the number of Pennsylvania Guard troops killed in action since 9/11, seven of whom have died in Afghanistan.
The last previous deaths were two soldiers in June of last year.
On Sunday, in an unrelated incident, a regular Army soldier from Bucks County - Sgt. First Class Kenneth B. Elwell of Holland - was killed in Afghanistan. He was one of two soldiers who died in a bomb blast. He was assigned to a unit of the 25th Infantry Division in Alaska.
The 131st Transportation Company, which is headquartered in Williamstown, in a rural area of north-central Pennsylvania, was mobilized in November. Nicknamed the "Road Runners," many members came from Dauphin, Lebanon and Schuylkill Counties. The unit trained for a few weeks at Fort Dix and then flew to Afghanistan.
This was the company's third deployment for combat duty in the decade since the terrorist attacks of 2001.
Sgt. Matt Jones, a Guard spokesman, said some soldiers had also been deployed as individuals.
Their job - riding in armored vehicles on roads known for attacks with improvised explosive devices (IEDs) - was quite dangerous.
"This is the first time they have lost anybody, but it was kind of talked about," Jones said. "If you are going to be driving trucks on a daily basis, IEDs are going to be something you definitely think about."
Gov. Corbett, in a statement, said: "Our thoughts and prayers go out to the families of these brave young men who have made the ultimate sacrifice on behalf of this country."
Koehler was a 1982 graduate of Lebanon High School, the Guard said. After serving in the Marines from 1982 to 1988, he took a 10-year break from service. He enlisted in the Guard in 1997 as a motor transport operator, and served his entire Guard career in the 131st Transportation Company. He served with the unit in Kuwait and Iraq from 2003 to 2004.
"As a former Marine noncommissioned officer, Edward Koehler was known for his impeccable military bearing and eagerness to challenge himself and those around him," Maj. Gen. Wesley Craig, Pennsylvania adjutant general, said in a statement. "When he was a private in the Marine Corps, Koehler took pride in playing Taps at the end of each day while his company was deployed to Diego Garcia," in the Indian Ocean. "Unfortunately, it is now our solemn duty to play Taps for him."
Mowery was a 1980 graduate of Central Dauphin High School in Harrisburg. He was in the Marine Corps Reserve from 1979 to 1985. He joined the Guard 15 years later in 2000, becoming an infantry team leader and motor transport operator. He served for several months in Kosovo from 2003 to 2004.
Said Craig: "He demonstrated leadership potential far above his rank and never hesitated to assume responsibility, no matter how big the challenge."
VanGiesen was a 1999 graduate of Kane Area High School. He enlisted in 1999 as a fuel and electrical systems repair specialist. He later become a heavy-vehicle driver, light-wheeled vehicle mechanic, and armament repairman.
He was on his fourth active-duty tour. He served in Germany in 2002 and 2003, in Iraq in 2005 and 2006, and in Iraq again in 2009.
He was "an enthusiastic soldier who never shied away from the call to duty," Craig said. "During his 12 years in the National Guard, VanGiesen served more than four total years on active duty."
Contact staff writer Tom Infield at 610-313-8205 or firstname.lastname@example.org.
Tom Barnes of the Pittsburgh Post-Gazette contributed to this article.