An official alert and multiple tweets went out announcing major police activity at SF General Thursday afternoon, and an anonymous tipster wrote to SFist saying, "My friend works at SF General and says that they 'are getting hardcore evacuated right now,' have been for 15 mins. Currently in the parking lot of main building. Heard a loud bang and reports of shots fired. Deputies ran into new ER."

Initial reports and SFPD's spokesperson Ofc. Grace Gatpandan suggested there were possible shots fired, however subsequent reports of a loud bang, like an explosion, complicated the story.

NBC Bay Area reported that "The San Francisco Police Department and Sheriff's Department are searching parts of the San Francisco General Hospital after a fire alarm went off and a loud bag was heard. Nothing has been found yet."

The bomb squad arrived, and ABC 7 reported the FBI is on the scene as well, for reasons that were not clear.

As of 2:44 p.m., the SFFD public information officer tweeted that no one had been injured. CBS 5 reports that the situation began at 2:15 p.m., and while some hospital employees "self-evacuated," no patients have been evacuated.

Update 1: Via the police scanner, second-hand on Twitter, we have mention of a possible explosive device found on the 5th floor of the hospital.

The new wing of the hospital, now officially known as the Priscilla Chan and Mark Zuckerberg San Francisco General Hospital and Trauma Center, opened to patients just six weeks ago.

Update 2: Officials are now saying that the "barrel bomb" described on the scanner was actually just an illegal firework. Everyone calm down.

The Chronicle confirms that, saying illegal fireworks have been found (an M80), and that the earlier evacuation occurred in "the outpatient area of the hospital that contains the family health and urgent-care buildings."

Below, the original breaking tweets.