ABC7 News has learned that consultants hired by homeowners of Millennium Towers will begin their work two weeks from now.
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The foundation of the tower is supported by piles deep into dense sand but not into bedrock like many other buildings here. It turns out, some of the city's best known skyscrapers are also built the same way including the Embarcardero Center, the SF MOMA and the Marriott and 101 California.
WATCH VIDEO: Luxury skyscraper Millennium Tower sinking in downtown SF
"We want to have an assessment from our building inspection department as to each and every building that has these types of foundations," said San Francisco Supervisor Aaron Peskin .
That's the same concern from the Millennium's Homeowners Association, which has launched its own investigation.
ABC7 News has learned they've hired consultants, who'll begin seismic testing and sample drilling on their building September 26.
WATCH VIDEO: Residents meet to discuss sinking of Millennium Tower
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Earlier this week, Supervisor Peskin released a letter from city building inspectors written in 2009 - just a year after the building was completed.
They asked Millennium project engineers to address their concerns that the building was sinking more than expected.
Peskin says the response to the letter was missing from 1,600 pages of records he reviewed. But he tells ABC7 News a third party source just sent him a copy of that response from project engineers.
It confirms what he thought.
"Before the first unit was ever sold, we already had information that the building had sunk 8.3 inches and was projected to sink another two to four inches," said Peskin.
Now the residential tower is sinking 16 inches and perhaps more concerning, its tilting almost two inches.
Click here for a look at more stories and videos about the Millennium Tower in San Francisco.