Pictured: The 'lovable, likeable' professor shot dead by student 'for getting bad grades'

Police work at the scene of a shooting at the University of California, Los Angeles, Wednesday, June 1, 2016, in Los Angeles

Sheriff deputies work at the scene of a fatal shooting at the University of California, Los Angeles, Wednesday, June 1, 2016, in Los Angeles

In this image made from video, police respond to a fatal shooting at the University of California, Los Angeles, Wednesday, June 1, 2016, in Los Angeles

Police at the scene of a shooting at the University of California, Los Angeles (AP)

Los Angeles fire department personnel gather at the scene of a fatal shooting at the University of California, Los Angeles (AP)

UCLA (Inset: William Klug)

Police work at the scene of a shooting at the University of California, Los Angeles, Wednesday, June 1, 2016, in Los Angeles

Sheriff deputies work at the scene of a fatal shooting at the University of California, Los Angeles, Wednesday, June 1, 2016, in Los Angeles

Police work at the scene of a shooting at the University of California, Los Angeles, Wednesday, June 1, 2016, in Los Angeles

In this image made from video, police respond to a fatal shooting at the University of California, Los Angeles, Wednesday, June 1, 2016, in Los Angeles

Police at the scene of a shooting at the University of California, Los Angeles (AP)

Los Angeles fire department personnel gather at the scene of a fatal shooting at the University of California, Los Angeles (AP)

UCLA (Inset: William Klug)

Christopher Weber and Christine Armario, in Los Angeles

June 02 2016 06:54 AM

The victim of a murder-suicide that locked down the University of California, Los Angeles for hours has been identified as a mechanical engineering professor.

William Klug was gunned down in an engineering building office on Wednesday morning, according to a law enforcement source with knowledge of the investigation. The gunman has not yet been identified.

It is being reported by US media that he may have shot his professor dead because of "bad grades".

Colleagues of Professor Klug said he was a married father of two and a kind, gentle person.

GoFundMe started for family of William Klug, professor killed in UCLA murder-suicide https://t.co/v6cVoa8mCg pic.twitter.com/ZKQP5LPoZd
— ABC7 Eyewitness News (@ABC7) June 2, 2016

UCLA biology and chemistry Prof Charles Knobler said those who knew Prof Klug were in shock and described his colleague as "a very lively, lovable, likeable guy".

Hundreds of heavily-armed police swarmed across the sprawling campus as thousands of staff and students barricaded themselves in classrooms and offices, some using belts and chairs to secure doors, until authorities determined the gunman and victim were dead.

Police work at the scene of a shooting at the University of California, Los Angeles, Wednesday, June 1, 2016, in Los Angeles

The two men died in an engineering building office and authorities found a gun and what might be a suicide note, Los Angeles police chief Charlie Beck said.

Two people shot dead in murder-suicide as US university goes into lockdown

Groups of officers stormed into buildings that had been locked down and cleared hallways as police helicopters hovered overhead.

Sheriff deputies work at the scene of a fatal shooting at the University of California, Los Angeles, Wednesday, June 1, 2016, in Los Angeles

Advised by university text alerts to turn out the lights and lock the doors where they were, many students let friends and family know they were safe in social media posts.

The shooting happened in the week before final exams at the Los Angeles campus, whose 43,000 students make it the largest in the University of California system. Classes are resuming on Thursday.

Police work at the scene of a shooting at the University of California, Los Angeles, Wednesday, June 1, 2016, in Los Angeles

Olivia Cabadas, a 22-year-old nursing student, was getting ready to take a quiz in the mathematics building when her classmates began getting mobile phone alerts. Through a window, they could see students rushing down the hallway.

An officer yelled that everyone should get out.

William Klug, professor killed in UCLA murder-suicide

"It was just a little surreal - this is actually happening," Ms Cabadas said. "It was chaos."

Those locked down inside classrooms described a nervous calm. Some said they had to rig the doors closed with whatever was at hand because they would not lock.

Umar Rehman, 21, was in a maths sciences classroom next to Engineering IV, the building where the shooting took place. The buildings are connected by walkway bridges near the centre of the 419-acre campus.

"We kept our eye on the door. We knew that somebody eventually could come," he said, acknowledging the terror he felt.

The door would not lock and those in the room devised a plan to hold it closed using a belt and crowbar, and demand ID from anyone who tried to get in.

Scott Waugh, an executive vice chancellor and provost, said the university would investigate concerns about doors that would not lock, but overall, the response was smooth.