Speaking for the first time since Cho Seung-hui killed 32 people and himself on Monday, the family said it has been "living a nightmare" since the attack.

In a statement Cho's sister, Cho Sun-Kyung, spoke of the family's pain following the massacre.

"Our family is so very sorry for my brother's unspeakable actions. It is a terrible tragedy for all of us, " she said.

On behalf of her family Ms Cho, a 2004 Princeton University graduate who works as a contractor for the US state department, expressed grief for the friends and family of her brother's victims.

"We never could have envisioned that he was capable of so much violence.

"We pray for their families and loved ones who are experiencing so much excruciating grief. And we pray for those who were injured and for those whose lives are changed forever because of what they witnessed and experienced, " she said.

"Each of these people had so much love, talent and gifts to offer, and their lives were cut short by a horrible and senseless act."

Ms Cho said the family felt helpless, lost and shocked by her brother's actions.

A six-member independent panel . . . including former homeland security chief Tom Ridge . . . is to examine how authorities reacted to the crisis, amid claims officials ignored warning signs that Cho, who had been admitted to a mental health unit in late 2005, was a danger. Police spokeswoman Corinne Geller said investigators hoped to have something to tell the public next week.