A stolen firearm used in the mass shooting in Sacramento on Sunday was converted to be used as a fully automatic weapon, police said.

Authorities did not offer more details about the weapon. But it is possible to convert a handgun into a automatic weapon that can fire like a machine gun.

There had been suspicion that an automatic weapon was used in the shooting because video of the incident recorded rapid fire. When the gunfire was over, 18 people had been wounded, six fatally.

Officials estimate at least 100 shots were fired.

The stolen gun used in the Sacramento mass shooting was converted into a fully automatic weapon for maximum firepower, police say.

Investigators have obtained a social media post made on Saturday by one suspect, Smiley Martin, in which he wields the stolen gun, according to law enforcement sources.

That post has since been removed. The Times didn’t independently review the post.

Sacramento police announced Martin’s arrest Tuesday. In a statement, they said Martin, 27, was among the seriously injured and remained hospitalized under police custody. He will be booked at the Sacramento County Jail on suspicion of “possession of a firearm by a prohibitive person and possession of a machine gun” as soon as his medical care is completed, police said.

Martin’s brother, Dandrae Martin — previously identified by police as Dandre Martin — was arrested early Monday. The 26 year old was booked on suspicion of assault with a firearm and being a felon in possession of a gun.

Videos have emerged of the shooting in downtown Sacramento that underscore the chaos of the situation.

More than a day after the Sacramento mass shooting, many questions remain unanswered. Here’s what we know.

One shows a large group of people fighting on the street, followed by rapid gunfire. Another captures multiple volleys of gunfire, including what sounds like a possible automatic weapon, before people on the street run for their lives.

Sacramento police said they believed a fight might have sparked the gunfire but were unclear about what prompted the conflict.

Witness Alexandra Arellano said she was about to walk outside of a club where she works when she heard one gunshot and then “around like 30 or 50 rounds being shot.”

When the shooting ended, six people were dead and 10 had been wounded in the heart of downtown Sacramento’s entertainment district.

Arellano’s fiance, Jesse Fuentes, said he and another security guard heard a commotion at a nearby garage.

“Once we went over there, it was pretty much a gunfight going on,” he said. “We were just trying to take cover because we couldn’t tell where the shots were coming from at first, because they were coming from two different areas. But the one that really just freaked everybody out was the automatic weapons. That’s when everyone was running and pushing.”

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Richard Winton is an investigative crime writer for the Los Angeles Times and part of the team that won the Pulitzer Prize for public service in 2011. Known as @lacrimes on Twitter, during almost 30 years at The Times he also has been part of the breaking news staff that won Pulitzers in 1998, 2004 and 2016.