A third son is to return from Iraq, a police spokesman said on behalf of the family.
The family of Spc. Nathan Hubbard, 21, was taking his death "very, very hard," said police spokeswoman Janet Stoll-Lee. The soldier's father, Jeff Hubbard, is a retired 30-year veteran of the police department.
Nathan's older brother, Marine Lance Cpl. Jared Hubbard, died in a roadside bomb explosion in downtown Ramadi in 2004. A third brother, Jason, will be returning home from Iraq to be with his family, Stoll-Lee said on behalf of the Hubbards.
The helicopter went down during a nighttime mission in the Tamim province that surrounds Kirkuk, an oil-rich city 180 miles (290 kilometers) north of Baghdad, said Lt. Col. Michael Donnelly, a military spokesman in northern Iraq.
He said facts gathered indicated it was almost certainly due to a mechanical problem and not hostile fire, although the final cause remained under investigation. The military did not immediately release the soldiers' identities pending notification of relatives.
Nathan Hubbard was assigned to the 25th Infantry Division in Hawaii, where officials said 10 of the soldiers killed in the crash were based.
Keith Butterfield, a family friend, said the Hubbards were worried for their sons but proud because they knew they were committed to going on behalf of their fallen brother.
"There is nothing anyone can say to make it better, but it's good to know that there are other families that can help you cope," said Butterfield, who became close to the Hubbards after his own son died in Iraq last year. "It's bringing up the feelings of everyone else's loss, but we will be there for them."
Nathan and Jason Hubbard joined the Army together in 2005, shortly after their brother was killed. Their mother, Peggy, told the Fresno Bee newspaper in a 2005 interview that she believed Jason joined in part to protect Nathan after not being there to help Jared.
The brothers said at the time that they didn't worry about dying in the war.
"People are going to be hurt, and people are going to be killed," Nathan Hubbard told the Bee. "That is a reality you have to accept, but not dwell on."
The choice of the city of Helsinki is not incidental as the capital of Finland had hosted US-Soviet negotiations on the limitation of nuclear stockpiles in 1969