US police Saturday sought to uncover why a jobless immigrant snapped, going on a murderous rampage in the centre where he learned English, mowing down 13 people before killing himself.

The bespectacled gunman at the heart of the tragedy in the quiet New York town of Binghamton was identified as Jiverly Voong (41) of Vietnamese descent, who until early March had been taking classes at the American Civic Association.

Early on Friday Voong, who had legally changed his name from Wong, donned body armor, blocked the centre's back doors with his car and then burst into the front of the building in a hail of gunfire, police said.

Without uttering a word, Voong shot two receptionists, killing one, then strode into a classroom where an English lesson was being held and shot dead another 12 people, injuring three more, before turning the gun on himself, police chief Joseph Zikuski said.

The alarm was raised by the surviving receptionist, hailed as a hero by the police chief, who played dead after being shot in the stomach. She then crawled under her desk to call 911 on her cell phone.

She and the other three injured were Saturday still being treated in hospital. Two were in a critical condition.

Community shocked

The small community, 215 kilometres northwest of New York city, voiced shock on Saturday at the outburst of violence, especially angered that it had happened at a centre trying to help new immigrants chasing the American dream.

"That this tragedy should have happened in our community to our friends who only wanted to advance their knowledge and love of America is unbearable," the center's president Angela Leach told reporters as she fought back tears.

"Whatever drove this individual to do what he did, I cannot possibly fathom," she added, vowing to continue the center's work "to help people realise the dreams of American citizenship."

The names of the victims have not yet been released, but mayor Matthew Ryan said Binghamton authorities had had inquiries from nine countries and two consulates about the safety of their nationals.

Unable to find a job

Voong, who had a gun permit since the late 1990s, had recently lost his job at the Shop Vac assembly plant, which the New York Daily News reported had closed down in November.

"We picked up that apparently people were making fun of him. He felt he was being degraded because of... his inability to speak English. And he was upset about that," Zikuski said.

A former co-worker of Voong quoted in The New York Times said he was surprised to see Voong at a local gym three times, including the night before the shootings.

Son Quach said Voong told him in Vietnamese he had been laid off with $200 a week in unemployment benefits, and that he spent more time in the gym because he couldn't find another job.

Police said they had interviewed the killer's parents and sister, with whom he shared a home in neighboring Johnson City.

"Obviously this investigation will focus a lot upon what the motive may have been. We may not ever come up with anything," Zikuski added.

But he did rule out any tie to terrorism, after a militant Taliban leader wanted by the Pakistani government claimed responsibility for the massacre.

Second mass shooting in a week

It was the second mass shooting in less than a week in the United States, as the economic downtown claims more jobs and blights more lives.

On Saturday, a 23-year-old man shot and killed three policemen in Pittsburgh, Pennsylvania after apparently losing his job at a glass works.

Binghamton too has been hit by layoffs, with insurer AIG and computer giant IBM both cutting staff.

President Barack Obama, speaking in Strasbourg on the sidelines of Nato summit, said: "I am heartbroken for the families who survived this tragedy.

"It just underscores the degree to which in each of our countries, we have to guard against the kind of senseless violence that tragedy represents."

Friday's carnage is the latest incident to rock small-town America, where many fiercely defend the legal right to bear firearms, but which is also being hit by the worst economic crisis since the Great Depression.

On Sunday, a heavily-armed man burst into a North Carolina nursing home, where his estranged wife worked, killing eight people before being shot and wounded by a policeman.

It also comes days before the second anniversary of a massacre at Virginia Tech ? the deadliest school shooting in US history in which 32 students and professors were shot dead by a student gunman ? and weeks before the 10th anniversary of the Columbine, Colorado school shooting.