By Douglas Hanks, Bianca Padro Ocasio, Martin Vassolo and David Ovalle, Miami Herald

Surfside, Florida – Exactly one week after the Champlain Towers South building collapsed, rescue workers stopped work early Thursday amid concerns that the remaining structure could topple.

Also, South Florida is bracing for the possible arrival of a tropical storm later in the week that could affect the site.

The new challenges arose Thursday as President Joe Biden prepares to visit Surfside, near Miami, to meet with rescue workers, console grieving families, and deliver remarks about what could be the nation's deadliest building collapse.

Biden is scheduled to arrive in South Florida Thursday morning along with the first lady. Residents and officials are hoping that Biden's visit offers comfort to the small condo community just north of Miami Beach, and the frustrated rescue workers piecing through the gigantic heap of twisted concrete and metal.

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"The president will at least energize them, so at least the families will know people care," said Surfside resident Antonio Pons, who lives near the collapse site.

So far, rescuers have recovered the bodies of 18 people, and say another 145 remain missing. Late Wednesday, Miami-Dade police identified two sisters, 10-year-old Lucia Guara and 4-year-old Emma Guara, as the latest victims in the condo collapse. Their parents also died.

At the site of the Surfside condo collapse, the massive rescue effort – featuring specialized emergency workers from around the country and as far away as Mexico and Israel – had been continuing around the clock, through stifling heat and frequent rainstorms.

But work reportedly stopped early Thursday morning after authorities voiced new, urgent concerns that the remaining structure of the 12-story Champlain Towers South could topple.

After 2 AM, multiple police officers and rescue personnel said they were hearing warnings the vacant building was shifting and the new instability could lead to another collapse. That led to clearing people from the area around the rescue operation, they said.

A crane that had been used to move debris all evening was stopped around 3 AM. Multiple police officers said work on the collapse site had halted and the area was cleared.

"It's on stand-by right now," one officer said. The two buildings on either side of the Champlain South were evacuated shortly after the June 24 collapse.

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The federal government has offered wide support, from helping victims relocate to dispatching scientists and experts to help determine what caused the 40-year-old building to suddenly collapse in the middle of the night, as residents and guests slept.

The US National Institute of Standards and Technology, which has been in Surfside since Sunday, said Wednesday that it will launch a full investigation into the building collapse, and what changes in laws, building codes, and regulations could be made to prevent a similar tragedy.

The same agency pushed for safety reforms after investigating the collapse of the World Trade Center's Twin Towers in 2001.