(CNN) A gunman in a white dress shirt and slacks walks briskly through the Fifth Third Center's lobby, a satchel slung across his shoulder, as he fires his handgun seemingly at whomever he comes across.

Less than five minutes after he starts his deadly attack, he tumbles to the ground, mid-stride, as police officers outside shoot him through a glass pane.

This is part of what's in an edited version of surveillance video that police released Friday, showing Thursday morning's deadly shooting on the ground floor of a Cincinnati skyscraper.

Omar Enrique Santa Perez, 29, walked into the lobby shortly after 9 a.m. and killed three men and injured two other people in the building off Cincinnati's landmark Fountain Square, authorities said.

Perez fired 35 rounds from a semiautomatic 9mm handgun in just under four and a half minutes. Investigators still are trying to figure out why, Police Chief Eliot Isaac said Friday.

"(He was) firing shots at anyone he sees," Isaac told reporters at a news conference.

The chilling surveillance video shows at least one moment of desperation and heroism. An unarmed security officer helps someone up and races with that person around a corner, moments before the gunman walks into view, firing shots.

Numerous people called 911, and four armed officers arrived at the center's exterior 55 seconds after the first call, Isaac said.

An officer's body-camera video shows him shooting into the glass about six times. Gunfire from other officers follows.

Surveillance video from inside the Fifth Third Center. pic.twitter.com/bi5fpmJHNE — Cincinnati Police Department (@CincyPD) September 7, 2018

Perez had about 250 rounds in the satchel, Isaac said.

If it had taken longer to kill him, Perez could have killed many more people, Mayor John Cranley said.

"The police saved lives, and we owe them all a huge debt of gratitude," Cranley said.

Hamilton County Prosecutor Joe Deters relayed a similar sentiment.

"This could have been a bloodbath beyond imagination," Deters said Thursday, according to CNN affiliate WLWT

Three men killed; woman shot 12 times

Killed were Luis Felipe Calderon, 48; Pruthvi Raj Kandepi, 25; and Richard Newcomer, 64, according to the Hamilton County Coroner's office.

Two other people were hospitalized, including a woman whose family says she was shot 12 times.

Whitney Austin, a Louisville-based senior product manager for Fifth Third Bank, was in Cincinnati for a meeting. She was on a conference call when she walked into lobby's revolving doors as the shooting began, the bank said in a statement.

Two officers "rushed to her aid and carried her out of the line of fire after she was shot," the statement reads.

"I'm still processing the events from yesterday, the impact to the victims, to me and the community. And I'm thinking about how I can make a difference in the future," Austin said in the statement.

"I'm so thankful to the paramedics and first responders and the doctors, nurses and staff at UC Medical (Center)," she said.

According to a spokeswoman for the University of Cincinnati Health system, one victim has been upgraded from critical to serious condition, and one remains in fair condition.

Gunman's motive remains unknown

Much is still unclear about the gunman.

He was not a current or former employee at the center, whose primary tenant is the Fifth Third Bank headquarters. Officials say they are not sure of his connection with any surrounding businesses.

Emergency personnel and police respond to a reported active shooter situation near Fountain Square on Thursday.

He lived in the Cincinnati area since about 2015, most recently in the nearby village of North Bend, Ohio, Isaac said.

Authorities were trying to determine what may have motivated him, but have said he acted alone.

A SWAT team and bomb squad investigators were searching Perez's apartment hours after the shooting to learn more about him. Police didn't release details about the search.

Perez had no criminal history in Cincinnati but was charged with misdemeanors in Florida and South Carolina, Isaac said.

The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives responded to the scene and are assisting Cincinnati police in their investigation, the bureaus said.

Victims

Newcomer, a superintendent at Gilbane Building Company, was working Thursday at the Fifth Third Center, Gilbane spokesman Wes Cotter said.

Newcomer had worked in construction for Gilbane for more than 30 years, and was involved in building many places where people in Cincinnati now live and work, Cranley said, citing a conversation with Newcomer's wife.

One of Newcomer's colleagues, Juan Manuel Montoya, was taking a break when the shooting started. As he helped others take cover, Montoya said Newcomer was injured in the building's basement and ran to get help from medics.

"I really feel sad that Rick didn't make it, but I did as much as I could to get help," Montoya told CNN affiliate WLWT.

Newcomer had worked for the company for four years, overseeing trade workers like Montoya.

"This is a shock that none of us could have ever expected and we're focused on praying for his family and getting through this," Cotter said.

Law enforcement respond to the scene of Thursday's shooting at Cincinnati's Fountain Square.

'I'm next, I'm next'

The gunman was among dozens of people visiting stores, business offices, hotels and eateries in Fountain Square on Thursday morning.

Police say he drove his own vehicle to the Fountain Square area sometime before 9 a.m. and entered several businesses, including a sandwich shop at the Fifth Third Center. He entered the center's lobby through that shop, police said.

Earlier, police said Perez had accessed the high-rise building through a loading dock. Isaac clarified Friday that two wounded people fled to the loading dock area, but Perez had not been there.

Workers quickly fled the building and hid in bathrooms and closets when they heard a hail of gunfire and smelled gun power.

"All that was going through my mind was they're coming, they're coming for us. The shots got closer and closer and closer and closer and that's all you can think about is, I'm next, I'm next, I'm next," Janetta Cook, who works at a bakery shop near the lobby, told CNN affiliate WCPO

Another employee, Michele Parks, said she was about to go into the bank when Cook stopped her.

"There was nowhere we could go," Parks told WCPO. "The only thing we could do was to protect each other, go in the bathroom and pray everything was OK."

One witness, James Walker told CNN affiliate WLWT, said he heard about eight gunshots and got on his knees.

"I kind of got down on the ground and then all the cops started coming," he said. "I'm still shaking."