Sen. Dick Durbin (D-Ill.) called for an investigation into how the man who shot Rep. Gabrielle Giffords (D-Ariz.) and as many as 19 others in Tucson, Ariz., Saturday obtained a handgun despite a “history of mental instability.”

The shooting has launched a fresh debate on gun control laws.

“How did he go through the process and end up with this gun and with this ammunition?” said the Senate majority whip on CNN’s “State of the Union.” “I think that’s a legitimate question.”