San Francisco's Millennium Tower, the 58-story downtown residential tower that has sunk around 17 inches and tilted to the northeast since the building opened in 2009, is back in the news following a cracked window that could be related to the way the building is shifting.

A drone sent up to inspect the cracked window on the exterior of the building's 36th floor crashed on Saturday, KPIX-TV reports.

The condo tower's homeowners association had hired the drone operator to take photos and video of the window, but the operator lost control of the drone, which hit a nearby building and then fell to the ground.

In a report to city officials last week, architectural engineering firm Allana Buick & Bers said the crack could be related to a localized defect or could be a more widespread problem related to the way the building is sinking, the San Francisco Chronicle reports.

The sinking tower spurred much finger-pointing and several lawsuits starting in 2016 when it was discovered that Millennium Tower was sinking and tilting, already exceeding the 12 inches the building was expected to sink over its lifetime. Millennium Partners was criticized for not having driven down to anchor the foundation into the bedrock below the building to secure it. The developer blames the Transbay Joint Power Authority, arguing that the work on the nearby Transbay Transit Center (the newly opened center is now dubbed Salesforce Transit Center) weakened the ground and caused the tower to sink. The shifting has also raised fire safety concerns.

Solutions proposed have included drilling 50 to 100 piles into the bedrock from the building's basement, or even building a larger, heavier structure beside the tower that would connect to and support the taller tower.

Saturday's drone images were part of the effort to better determine what caused the crack in the window.

Construction experts for Millennium Partners had determined in a prior assessment that the crack was an isolated issue, the Chronicle reports. The company plans to further inspect and replace the window.

For now, the window has been taped up from the interior and the homeowners association has built a structure at street level to protect passers-by, KPIX-TV reports.

Someone is expected to rappel down the side of the building next to inspect the window and tape up the glass from the outside, the Chronicle reports.