The brothers were stationed with the same unit in Iraq when a truck rigged with explosives detonated Sunday next to a convoy, killing Army Spc. Jeremy L. Ridlen, 23.
Guard officials wouldn't say if his identical twin, Jason Ridlen, was with the convoy at the time, but he is expected to accompany his brother's body home to Illinois.
Friends said Tuesday they worry about how he will deal with the loss.
"They just clung to each other. You can't talk about one without talking about the other," said Diane Daggett, one of the brothers' teachers at Maroa-Forsyth High School, where the twins graduated in 1998.
The same year, they joined the Illinois National Guard and were assigned to the 1544th Transportation Company.
Jeremy Ridlen was part of a convoy traveling a supply route in the Fallujah (search) area Sunday when a dump truck rigged with a bomb exploded, according to the Illinois National Guard.
The boys grew up in nearby Maroa, a central Illinois town of about 1,600 residents 30 miles south of Bloomington. Both were students at Illinois State University when their unit was activated last year.
Major Tim Franklin, a Guard spokesman, said it isn't unusual for relatives to serve in the same military unit.
When a soldier from Wisconsin was killed in Baghdad last month, her older sister was serving in the same unit and her twin sister was also in Iraq. Under Pentagon policy, when a soldier is killed while serving in a hostile area, close family members may request non-combat assignments. Both surviving Wisconsin sisters were reassigned.
The Rev. Marlin Jaynes, who is acting as a spokesman for the Ridlen twins' family, said it will be up to Jason Ridlen whether he returns to a combat zone in Iraq.
He said the twins' parents and sister do not want to talk to the media.
Jaynes said the twins were very active in the church, played on its softball team and were included in its weekly prayer list after they deployed to Iraq. He said they looked so much alike that "I was not always sure which one I was talking to. They could trick me."
Their high school math teacher, Erin Morrison, said Jeremy Ridlen always tried hard in school.
"Teenagers these days can be really attitude-filled, but he didn't ever have an attitude. He was just a really good kid," she said.
"They were always together," she said. "I wasn't surprised that they were together over there as well."