Vice President Joe Biden called Saturday's shooting of 18 people, including Rep. Gabrielle Giffords (D-Ariz.), a "horrific attack" that is impossible to explain.

"We do not yet know the motivation behind these shootings. But what we do know is that there is simply no justification, no rationale for such senseless and appalling violence in our society," he said in a statement.

A gunman opened fired at a public event hosted by Giffords in front of a Tucson grocery store. A suspect is in custody. Among the confirmed dead is federal Judge John Roll and a young girl.

Giffords is said to be in critical condition, but the trauma director at the University Medical Center in Tucson said he was "very optimistic" following surgery.

"Gabby is one of the finest members of Congress I know," Biden said. "She is a principled leader and a consensus-builder. She has spent her time in office working her heart out to improve the lives of the people she represents. And she loves her husband and family above all else. On this tragic day, Jill and I are praying for Gabby's recovery, and we hold her family in our hearts."