Airman 1st Class Matthew R. Seidler died Thursday in the attack, which killed two other airmen. They were patrolling in Helmand, a southwestern province that remains a Taliban stronghold.

“When he joined the Air Force, he blossomed. He became himself,” said a cousin, Kalyn Masek, who last communicated with Seidler on Tuesday, his birthday. “I was really, really proud of him and the man that he’d become.”

Seidler, an explosive ordnance disposal technician, entered active duty in November 2009. He was assigned to the 21st Civil Engineer Squadron at Peterson Air Force Base in Colorado.

“He loved what he did” for the military, said Masek, who was surprised when Seidler told her that he was joining the Air Force. Being involved in the disarmament of explosives fed his intellect, she said, and excited him in a way that prior false starts in his professional life had not.

Seidler graduated from Westminster Senior High School in 2006. He took classes for a year in business administration at Stevenson University and then started in a multimedia design program at Carroll Community College before deciding to join the military.

“He was extremely smart,” said longtime friend Bryan Vana, who’d known Seidler since middle school. Vana said he was taken aback when Seidler asked him to be a reference for his Air Force admission, but said the decision made sense because military service would satisfy Seidler’s desire for new, evolving challenges.

Andrea Masek said she often played poker with her nephew. Poker and other strategy games were his favorite pastime, she said, and he had a serious demeanor at the table.

“He was very logical, analytical,” she said.

Seidler and Kalyn Masek, only a year apart in age, were “attached at the hip” growing up. When they were children, their families would go to Deep Creek Lake together, Kalyn Masek said, and the two of them would “cause trouble and get dirty” while playing hours on end.

Shy growing up, Seidler became an adventurous adult, she said. When they were young, she was always the one to bring him out of his shell. But after he joined the armed services, she said, he became the encouraging, outgoing one. On his Facebook page, where his father announced his death to friends and family, Seidler posted photos from trips he’d taken to Paris and New York and hiking and camping in the mountains. He also shared samples of his graphic design work and his preference for the Baltimore Ravens.

Seidler’s parents and brother live in Westminster, Kalyn Masek said.

In a statement, Lt. Col. Mark Donnithorne, his squadron commander, said Seidler’s role as an explosives disposal technician was vital to the operation.

“We will never forget Matt’s sacrifice and dedication to his critical, yet dangerous, mission,” he said.

“This is a tragic day for Team Pete, the 21st Space Wing, the 21st Civil Engineer Squadron and especially for Matt’s family,” Col. Chris Crawford, 21st Space Wing Commander, said in a statement Saturday. “We will come together to help Matt’s family and friends through their grief.”