The chief of police for the Dallas Independent School District says the student who shot four students when he opened fire at a Dallas high school this week was in and out of the building in less than two minutes and is believed to have been targeting a specific student.

The shooting at Wilmer-Hutchins High School earlier this week unfolded in less than two minutes and began when another student let the armed suspect inside through a locked side door, Dallas ISD Police Chief Albert Martinez confirmed Thursday.

Dallas ISD Police Chief Albert Martinez said he doesn’t know if the shooting was preventable, “when somebody puts their mind to it and we’re doing everything that we can.”