BERKELEY, Calif. (AP) -- A fourth-floor balcony of a California apartment building collapsed Tuesday, killing six Irish students and leaving  seven other people with serious injuries during a 21st birthday party, officials said.

All of the dead in the college town of Berkeley were in the U.S. on temporary work visas, Irish Foreign Minister Charlie Flanagan said in Dublin.

Police were working with fire and building officials to determine what caused the accident. No information was available on the identities of the victims.

Police received a noise complaint concerning a loud party in the apartment about an hour before the balcony collapsed at 12:41 a.m. but had not yet responded, said Officer Byron White, spokesman for the Berkeley Police Department.

The balcony, estimated to have been 5 feet by 10 feet, separated completely from the building and landed on a lower balcony.

"What the first responders said is it was quite disturbing," White said. "Anytime you have a tragedy like this it's quite awful."

Two Irish students asleep in the building said they heard a bang during the accident.

"I walked out and I saw rubble on the street and a bunch of Irish students crying," said Mark Neville, who has been in the U.S. for three weeks under the J-1 visa program.

"I just heard a bang and a lot of shouting," added Dan Sullivan, 21.

The building in the heart of downtown Berkeley has apartments in the upper floors and retail shops at ground level. The walkable neighborhood just blocks from the University of California, Berkeley, is dotted with museums, a city library, restaurants, coffee shops and chain stores.

The Library Gardens apartments were built in 2006, the Los Angeles Times reported.

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