The wife of the suicidal Texas pilot who slammed his plane into an Austin office building called the attack today an "unimaginable tragedy."
Sheryl Stack, who was mentioned in the hate-filled suicide note thought to have been left by her husband, issued a statement extending "my sincerest sympathy to the victims and their families."
"Words cannot adequately express my sorrow or the sympathy I feel for everyone affected by this unimaginable tragedy," she said.
Sheryl Stack said she would not answer any questions because of the ongoing investigation.
The sheer volume of flames and smoke pouring from the Austin office building after A. Joseph Stack slammed into with his plane has prompted authorities to investigate whether he had some kind of explosive on board, sources told ABC News.
Stack, 53, topped off his single engine Piper Cherokee with fuel before crashing into the IRS offices in a kamikaze mission designed to punish the government he believed wronged him.
The full tank of fuel is believed to have contributed to the force of the explosion and subsequent fire, which investigators believe was probably a deliberate tactic by Stack.
Despite the spectacular crash and fire that left the seven-story building a blackened hulk, only Stack and one other person are believed to have died. Officials have not yet identified the second victim, a federal employee -- but a man told ABC News Huntsville, Ala., affiliate WAAY-TV that the dead man was his brother, Vernon Hunter, 61, an IRS collection agent.
"He took my baby brother," said Harold L. Jackson, visibly welling up. "Oh boy, I don't want to break up. ... I can't do that. As a Christian person I have to say 'Forgive us as we forgive those,' and I have to forgive that man for doing that. But its very difficult."
Hunter, the youngest of five brothers, was adopted as a baby and kept his birth family's name, Jackson told WAAY, but that did not lessen the bond.
"We called ourselves the Jackson 5 -- the other Jackson 5," Jackson said of Hunter and his other brothers.
Stack's father-in-law, Jim Cook, said there had been worry in Stack's family very recently about Stack's state of mind regarding his tax problems.
"She had a bit of a problem with him," Cook said, referring to his daughter, Sheryl. "And evidently, in the last few days ... it boiled in his brain. She informed us that she thought he was crazy."
He said his son-in-law had been "a good guy," and seemed normal when he saw him at Christmas time.
"Everything was fine," Cook said. "We went out and ate. We spent time in their house. They have a beautiful home."
Sheryl Stack did not address the suicide note investigators believe her husband wrote -- a lengthy rage-filled diatribe against the IRS, President George W. Bush and his own accountant.
She instead asked for "privacy and the personal space we need to get beyond the events of yesterday."
The suicide note was posted Thursday right around the same time as the 10 a.m. crash. It was signed "Joe Stack (1956-2010.)"