BERKELEY, Calif. — Victims' loved ones began arriving in Berkeley from Ireland as investigators Wednesday tried to pinpoint the cause of a balcony collapse that left six people dead and seven seriously injured.

The accident happened early Tuesday, during a 21st birthday party involving a group of young Irish and Irish-American men and women spending the summer in California.

"My heart breaks for the parents," Prime Minister Enda Kenny said from Dublin.

The small fifth-floor balcony was crammed with 13 people when it broke loose from an apartment building, dumping partygoers about 50 feet onto the pavement. At least one engineer said the balcony may have been overloaded.

Five 21-year-old students from Ireland were killed, along with a young woman from California.

Meanwhile, memorials took shape on the ground below the fallen balcony, with people leaving flowers, a pack of cigarettes, a photo, condolence notes and an Irish flag.

The Irish students were in the country on what are known as J-1 visas, which enable young people to work and travel in the U.S. over the summer.

The San Francisco Bay Area is especially popular with Irish students, about 700 of whom are working and playing here this summer, according to the consul general. Many work at Fisherman's Wharf and other tourist sites.

"For many of my countrymen, this is a favorite experience, and to have this happen at the start of the season has left us frozen in shock," said Philip Grant, Ireland's San Francisco-based consul general.

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 of Ireland.

As news of the accident broke, parents in Ireland desperately called the San Francisco Bay area to find out if their children were OK.

Sam McCarthy, 21, who was not at the party, said his mother called at 4 a.m. He said he was shocked by the news — he had come to California for a fun, independent summer.

"I thought it would be a cool experience," said McCarthy, who works at a bike rental business in San Francisco.

Investigators are expected to look at such things as whether the balcony was built to code, whether it was overloaded and whether rain or other weather weakened it.

Berkeley officials said the building code at the time of the apartment house's construction in 2007 required the balcony to hold at least 60 pounds per square foot. That requirement has since been raised to 100 pounds.

City spokesman Matthai Chakko said officials have not measured the balcony and how much weight it was built to bear. Chakko also said there is no city requirement to post a weight restriction for apartment balconies.

Grace Kang, a structural engineer and spokeswoman for Pacific Earthquake Engineering Research Center at Berkeley, said the balcony looked to her to be 4 by 6 feet, or 24 square feet.

That would mean it should have been able to bear at least 1,440 pounds. Thirteen normal-size adults would weigh more than that.

"They were packed like sardines, and then they were moving," she said. When people are moving, it "may further exacerbate" the strain.

Associated Press writers Olga R. Rodriguez, Paul Elias, Ellen Knickmeyer and Lisa Leff in San Francisco, Christopher Weber in Los Angeles and Bob Seavey in Phoenix contributed to this report.