A former Korean-American student accused of killing seven people at a private Christian college in California lined up his victims and shot them execution-style, police said Tuesday.

The 43-year-old, who planned the killings after being expelled from the college near San Francisco for "anger management" issues, was "upset" at staff and other students and had displayed "no remorse," a senior officer said.

Police were holding the suspect in the fatal shootings that took place Monday at Oikos University in Oakland, which stunned the tightly knit Korean-American community in the area.

"This was a calculated, cold-blooded execution in the classroom," Oakland police chief Howard Jordan told CNN.

The suspect, later identified as Goh Su-nam and had been using the name "One L. Goh" ever since moving to the US, opened fire with a .45 caliber handgun, according to local media reports.

"We learned from the suspect and witnesses he was distraught because he was picked on. ... He planned (the attack) several weeks in advance," Jordan told the local Fox affiliate KTVU.

Goh had difficulty speaking English, Jordan said, adding "he was so upset he went out and purchased a weapon and had every intent to kill people yesterday."

Although cooperating with officers, "he has not shown any remorse," Jordan said.

As well as the massacre scene, investigators focused Tuesday on a location a few miles away where they were searching "for firearm, believed to have been used and discarded yesterday by suspect," according to the police Twitter feed.

Details of the shooting emerged a day after the gunman allegedly walked into a building housing Oikos University, took a receptionist hostage and then sought out a particular female administrator.

When he realized the administrator was not in the building, he shot the secretary and then lined students up against a wall and shot them one by one, Jordan told CNN.

"I'm going to kill you all," the gunman allegedly told the students.

Six women and a man - all students at the school, aged 21 to 40 - were killed in the rampage. Jordan said they were from Nigeria, Nepal and Korea.

"This happened within minutes," Jordan said. "We don't think the victims had any opportunity to resist, any opportunity to surrender."

The gunman then walked out of the classroom, reloaded his automatic weapon, and fired into several classrooms before driving off in a victim's car to neighboring Alameda, California, he said.

He said the gunman then called his parents and surrendered to police who arrived on the scene.

"We've learned this was a very chaotic, calculated and determined gentleman that came there with the specific intent to kill people," Jordan said.

Some 35 people were in or near the building at the time. Of those, 10 were hit and five were pronounced dead at the scene. Two others died later in hospital. Survivors were found hiding in locked and darkened rooms.

The rampage appeared to follow a period of tumult for Goh.

A US army spokesman said that Goh's brother, Staff Sergeant Su Wan-ko, was killed in a car crash in Virginia in March 2011. Local news accounts at the time said he died after smashing into a boulder that had fallen onto the highway.

Oikos professor Soo Nam-sung told Reuters that Goh, who had studied nursing at the college, was despondent over the death of his mother about a year ago.

"He had gone through a difficult time to get good grades through his personal problems and he couldn't focus on his work," she said through an interpreter at a news conference.

Adding to his troubles, Goh had been involved in a dispute with the owners of an apartment in Virginia who had evicted him and claimed he owed back rent, court records showed.