Oklahoma hospital gunman killed four, injured several

(Tulsa, OK) -- New details are emerging as police piece together a mass shooting in Oklahoma.

They say the gunman knew exactly where he was going when he walked into a Tulsa hospital Wednesday with a rifle and a pistol, opening fire in a doctor's office on the second floor. Officers responded within three minutes and opened the door to find three workers and the shooter dead. A fourth victim died at the hospital.

Several others were wounded in the shooting.