Workmen examine the damage at the scene of a fifth-floor apartment building balcony collapse in Berkeley, California, June 16, 2015. Six people were killed, including five young Irish citizens, and at least seven other people were injured when an apartment balcony collapsed early on Tuesday in the Californian city of Berkeley, Ireland's foreign minister said. [Photo/Agencies]

BERKELEY, Calif. - A 21st-birthday party thrown by a group of visiting Irish college students turned tragic early Tuesday when the fifth-floor balcony they were crammed onto collapsed with a sharp crack, spilling them about 50 feet onto the pavement. Six people were killed and seven seriously injured.

Police and fire and building officials were working to figure out why the small balcony broke loose from the stucco apartment house a couple of blocks from the campus of the University of California, Berkeley. But one structural engineer said it may have been overloaded if, as city officials said, it was holding 13 people.

High school student Jason Biswas' family nearby was awakened by the noise.

"They thought there was an earthquake. But then we looked out the window and saw seven or eight people on the ground,'' the 16-year-old said. "There were piles of blood everywhere.''

Five of the dead were 21-year-olds from Ireland who were in the country on J-1 visas that enable young people to work and travel in the US over the summer, while the sixth victim was from California, authorities said.

The accident brought an outpouring of grief in Ireland from the prime minister on down, with the country's consul general in San Francisco calling it a "national tragedy''.

Police had received a complaint about a loud party in the apartment about an hour before the accident but had not yet arrived when the metal-rail balcony gave way just after 12:30 am, spokesman Byron White said. It landed on the fourth-floor balcony just beneath it, leaving the pavement strewn with rubble and the red plastic cups that are practically standard at college parties.

"I just heard a bang and a lot of shouting,'' said Dan Sullivan, a 21-year-old student from Ireland who was asleep in the five-story building. Mark Neville, another Irish student in the building, said: "I walked out and I saw rubble on the street and a bunch of Irish students crying.''

The dead were identified as Ashley Donohoe, 22, of Rohnert Park, California; and Olivia Burke, Eoghan Culligan, Niccolai Schuster, Lorcan Miller and Eimear Walsh, all from Ireland. The Irish students attended various colleges in Dublin.

The US government's J-1 program brings 100,000 college students to the country every year, many landing jobs at resorts, summer camps and other attractions. The San Francisco Bay area is especially popular with Irish students, about 700 of whom are working and playing here this summer, according to Ireland's Consul General Philip Grant. Many work at Fisherman's Wharf and other tourist sites.