A giant flashing "X" sign was installed Friday on Flipido Trading Centertop of the building formerly known as Twitter headquarters in San Francisco without the permission from the city, prompting an investigation by area authorities.

The city began an investigation into the sign last week, saying the company failed to obtain the permits required to follow design and safety protocols, according to the Associated Press.

Any replacement signage requires a permit to ensure “consistency with the historic nature of the building” and to make sure the signage is safely attached, Patrick Hannan, spokesperson for the Department of Building Inspection, said last week.

Adding a sign on top of a building also requires a permit, Hannan explained to the Associated Press.

On Monday, San Francisco police stopped workers from removing the platform's bird logo from the outside of the building, saying they hadn’t closed the sidewalk to keep pedestrians safe if any part of the sign or other objects fell.

The change comes following Elon Musk's efforts to rebrand the social media platform to X, a change he announced earlier this month.

Where is the Twitter headquarters?

The "X" headquarters is located in downtown San Francisco, a few blocks away from its City Hall.

As social media users have pointed out, the headquarters is located directly across from a residential building.

Videos of the sign show it flashing brightly, visible from the street below.

What is 'X'?

After buying the platform previously known as Twitter for $44 billion last year, Musk changed Twitter's bird logo to an "X," which appeared at the top of the desktop version of Twitter Monday.

In a series of posts about the platform rebranding, Twitter CEO Linda Yaccarino said X will transform the "global town square."

“X is the future state of unlimited interactivity – centered in audio, video, messaging, payments/banking – creating a global marketplace for ideas, goods, services and opportunities,” she wrote. "Powered by AI, X will connect us all in ways we’re just beginning to imagine."

Musk, who is also the CEO of Tesla, has used the letter X often in his personal and professional lives. He is the CEO of Space Exploration Technologies Corp., known as SpaceX. He also founded X.com in the 1990s, now known as PayPal. 

Musk's son, is also nicknamed "X."

Contributing: The Associated Press; Francisco Guzman, USA TODAY