Police in Oakland, California say a former student who killed seven people Monday at a Christian college attended mainly by Korean-Americans had been upset with the school's administration for expelling him earlier in the year.

Oakland Police Chief Howard Jordan told CNN Tuesday that the South Korean-born One L. Goh entered Oikos University looking to take revenge on a particular female administrator.

Once inside, Jordan says Goh took a receptionist hostage and then entered a classroom, where he lined up random victims against the wall and shot them one by one. In addition to the fatalities, three people were injured.

Jordan called the incident a “senseless act.” He described the shooting in the classroom as a “calculated, cold-blooded execution.”

“There were several people hiding in locked buildings, locked doors, behind desks, as you can imagine very frightened, very scared. Some of them were injured, so we had to rescue them out. We had to force our way into a number of rooms.”

Jordan has said Goh was also upset with students for the way they treated him when he went to the school.

Student Deborah Lee said she heard the gunshots and fled.

“The first time when I heard it, I thought it was fireworks or just joking, but right after that one woman screamed a lot, shouting, and then my teacher went outside to check and then he said one woman told them, that somebody has a gun, run, and then we all of us in class just ran away.”

Oikos University founder, Pastor Jong Kim, tells the Oakland Tribune newspaper that the shooter is a former nursing student.

A spokesman for South Korea's foreign ministry said the government is monitoring reports about the incident, and called the shooting “unfortunate.”