Alameda County’s top prosecutor to investigate collapse.

Alameda County prosecutor vows a thorough review of fatal Berkeley case.

OAKLAND — Alameda County Dist. Atty. Nancy O’Malley promised Thursday a “thorough and exhaustive investigation” of the June 16 collapse of a Berkeley balcony to determine whether criminal charges or a civil consumer protection action are warranted.

Flanked by her chief assistant, O’Malley said at a news conference that her office had been monitoring the deadly collapse and moved to take on the lead role in a criminal investigation after Berkeley city officials announced Tuesday that they were not pursuing one.

The tragedy that left six Irish students dead — one of them a dual citizen from the Bay Area — and injured seven “has devastated communities throughout the Bay Area and Ireland,” she said.

Addressing the families, O’Malley said: “Each of you deserve to have this matter thoroughly and exhaustively investigated. We will do so. That is the pledge I make from my office.”

D. A. spokeswoman Teresa Drenick said that, to her knowledge, Alameda County prosecutors have never brought a criminal prosecution in the case of a death resulting from a construction defect, though other jurisdictions have.

The students were celebrating a birthday when the fifth- f loor balcony at the Library Gardens complex on Kittredge Street collapsed.

O’Malley declined to comment on potential targets of the investigation. The original developer, TransAction Companies, became Equity Management Group before selling the completed Library Gardens in 2007 to a property fund managed by the global f irm BlackRock. The complex is managed by Houston- based Greystar. The contractor was Pleasanton- based Segue Construction Inc. San Jose- based R. Brothers Inc., a licensed waterproofing contractor, lists Library Gardens on its website as one of its projects.

In written statements, a BlackRock spokesman pledged to cooperate and said company officials “strongly support the efforts of authorities to conduct a thorough investigation.” He said BlackRock has been working with the property manager, the city and independent experts “in a thorough review of the situation,” and as the advisor to the fund that invested in the building in 2007, “we are equally interested in fully understanding the cause of the accident.”

A Greystar spokesperson said that company also supports the inquiry and “will continue to aid authorities as they work to determine the cause of the accident.”

Trevor R. Martin, an outside spokesman for Segue, said, “We continue to offer our full cooperation to all investigating authorities.” A woman who answered the phone at R. Brothers Inc. declined to comment.

Berkeley officials Tuesday proposed strengthening local codes in an effort to prevent water intrusion and resulting dry rot in balconies. But they declined to conduct a materials analysis of evidence or draw conclusions about a cause.

O’Malley declined to weigh in on the city’s earlier decision to wrap up its findings, saying that “their investigation was limited in scope, and they made that clear in statements to the media.”

The collapsed f ifth- f loor balcony is in custody of the Berkeley Police Department, which, along with other city departments, is cooperating with the district attorney’s office, she said. The Sheriff ’s Department is working with the property owner to obtain custody of a second balcony, removed from the f loor below the collapse, and “moving it to a secure place so our forensic experts can analyze it.”

She declined to say where it has been but stressed that she was confident that the chain of custody has not been breached and that an exhaustive investigation can be carried out “with the evidence we have.”

She promised a thorough review but said criminal charges may not be warranted. Criminal negligence must involve more than inattention or mistakes in judgment, she said. Defendants must have acted in a reckless way that created a high risk of injury or death.

“It must be aggravated, gross, reckless, culpable,” O’Malley said, “and such a departure from what would be the conduct of an ordinary person as to be incompatible with protecting life.”

The gravest possible charge would be involuntary manslaughter.

Any findings of criminal negligence must be deemed provable in court for charges to be brought, she added. A thorough vetting “is what every victim deserves,” she said. “We grieve with them.”

Philip Grant, the San Francisco- based Irish consul general for the West, promised to assist O’Malley in “any way we can,” and said she had “reached out to the families” before her news conference.

“We continue to monitor,” he said, “and we appreciate that any investigation that will be taking place is an independent one.”