Seth Ator, the suspect alleged to have killed seven people in a shooting rampage Saturday, was fired from his job and called 911 and the FBI tip line in the hours before opening fire in and around the West Texas city of Odessa, authorities said Monday.

Ator went to work at Journey Oilfield Services on Saturday and was there a short time before he was fired, Odessa Police Chief Michael Gerke said in a news conference. Ator called 911, complaining about his employer, but left before officers responded.

Later,...