Miami, Jun 29 (IANS): The death toll from the collapse of a residential building in Surfside, the US state of Florida has increased to 11, Miami-Dade County Mayor Daniella Levine Cava said.

During a news conference on Monday evening, Cava said that the number of people accounted for now stands at 136, and that rescuers are still searching for 150 people who remain unaccounted for, reports Xinhua news agency.

The numbers are "very fluid and they will change", she said.

"We are continually auditing the list and getting more calls and information from family members. Please stand by for more information."

Appearing on CNN later, the Mayor vowed to "get to the bottom of what went wrong" as Surfside officials have hired a structural engineer to study the cause of the disaster.

"Unless it is a plane or a bomb that triggered this whole thing, sometimes you can't get it down to one cause," said Allyn Kilsheimer, the engineer who has been hired.

"You don't know what you're going to end up with until you finish the whole study."

Asked about the timeframe for completing the study, Kilsheimer said "there is no way" he was able to give a definitive one.

As investigations into the cause of the collapse are still ongoing, a 2018 report has emerged, in which engineers raised concerns about the structural damage of the condo built in 1981.