Three soldiers from a Baltimore-based Maryland National Guard unit were killed Friday in Iraq when an 18-wheeler struck the back of their armored vehicle, sparking a fire and detonating ammunition.

The Defense Department yesterday identified the three soldiers as Sgt. Brian R. Conner, 36, of Baltimore, Spec. Samuel M. Boswell, 20, of Fulton in Howard County, and Spec. Bernard L. Ceo, 23, of Baltimore. Baltimore officials said that Conner also had been a lieutenant with the Baltimore City Fire Department.

They were the first members of the Maryland National Guard killed overseas in the line of duty since World War II, said Maj. Charles S. Kohler, a spokesman for the Maryland National Guard. Guard officials and family members alike spoke of the severity of the loss.

The guardsmen were assigned to the 243rd Engineer Company, a 169-member unit that has been in the region since August to transport supplies, Kohler said. According to the Defense Department, the accident happened while the unit was conducting convoy operations in Taji, just north of Baghdad.

"This is the day we had been dreading," Kohler said. "We are a close-knit family, and we have suffered a great loss."

Kohler said that members of the Maryland National Guard have served in Iraq continuously since the beginning of the war. There are 607 members there now.

Maj. Gen. Bruce F. Tuxill, commander of the Maryland National Guard, released a letter he wrote to fellow soldiers about the accident.

"It is difficult to express the deep sadness that all of us in the Maryland National Guard family are feeling at this moment," he wrote. "Each one of these fine young men are true heroes in every sense of the word and represent the finest that the Maryland National Guard has to offer."

Boswell, who had recently turned 20, joined the Guard in 2003.

"I think everybody is in shock right now," a sister-in-law said last night.

Jackie Boswell said her brother-in-law had gone to Iraq willingly and voluntarily. She and her husband, Tony, drove him to the airport during Labor Day weekend on the last night he was home before going overseas, she said.

"He said he felt this is something he had to do and his country needs people to do it and he was going to do it," she said. "He felt it was his duty to do that."

Jackie Boswell said her brother-in-law, a "happy and happy-go-lucky" young man, was the youngest of eight brothers and sisters and, perhaps as a result, was "never stressed about anything." He also loved being around his 11 nieces and nephews, she said.

She said he graduated from River Hill High School in Clarksville and hoped to become a civil engineer.

"Our family was very close," she said. "It's very hard."

Ceo, who worked full time with the Guard, joined in 2001 and had served with the Honor Guard, Kohler said.

An aunt described him as someone "committed to doing what he thought would be right in terms of serving his country."

The aunt, Denise Ceo, said Bernard Ceo was the youngest of three sons and that his mother, Rosemary, "was very, very proud of him.

"That was her baby," Ceo said. He was "very, very nice -- very loving, very committed" and "very loyal to his family."

She said that her sister-in-law had said that she " 'wanted my baby to come home.' But God wanted him" also, and "he went where God wanted him to go.

"She finds solace in that idea," Ceo said.

A spokeswoman for Conner's family members said they were too distraught to comment.

Colleagues in the Baltimore Fire Department remembered Conner as a well-respected, conscientious firefighter and a good friend.

"He was a dear brother in the service. We have extended our heartfelt condolences to his family," said Kevin Cartwright, chief of public information for the Baltimore City Fire Department.

Conner, who joined the department in 1993, was described as "a gentle giant" -- he stood about 6-foot-2 -- who was close to his mother, Hortense Conner, and his siblings. He was also an adoring father to three daughters whom he was raising on his own, said Lt. Malik Habeebullah of Engine Company 20 in the Walbrook neighborhood, where they had worked together.

"That was his world. That was his everything," Habeebullah said.

He said Conner left Baltimore in June for training.

Word of his death reached Conner's fire department company by Friday afternoon. It was confirmed later that day by the department, devastating co-workers who had stayed in touch with him frequently by e-mail and through periodic calls Conner made to the station.

They had mailed him a large care package last Monday after hearing news that his supplies had been blown up, Habeebullah said.

The firefighters in the company are "like an extended family," Habeebullah said. Now, "they are leaning on each other for support."

Staff researcher Donald Pohlman and staff writer Rosalind S. Helderman contributed to this report.

Sgt. Brian R. Conner was raising three daughters.