By Eric Levenson, CNN

(CNN) — The New York City office building that was the site of a mass shooting Monday had significant security measures in place.

There was an off-duty police officer present, along with other security officers, a lengthy check-in process, locked turnstile gates, lobby alarms, elevators that allowed access only to certain floors, and, as a last resort, reinforced safe rooms.

Despite those measures, a 27-year-old armed with an assault-style rifle was able to force his way into 345 Park Avenue on Monday evening, killing four people before turning the gun on himself.

The shooting in the fortified Midtown Manhattan building has raised questions about office building security and whether more can be done to strengthen it.

“I don’t know how you get more secure than that,” New York Gov. Kathy Hochul said of the building, noting in particular the presence of a uniformed NYPD officer. “How you stop someone who walks in intent on murdering the first person they encounter, who is a police officer trying to stop them, that’s almost impossible. So people blaming the security system are looking at the wrong culprit here.”

Ken Wheatley, a security management consultant and founder and principal advisor of Royal Security Group, told CNN every office must conduct its own risk assessment based on its specific threat environment.

“You can’t prevent stuff from happening 100% … unless you’re Fort Knox, and there’s a lot of downsides to having an environment like that,” Wheatley said. “You make your best effort, you do all the things that we talk about, and you hope that’s sufficient to prevent something from happening.”

How the shooter took on office security

The shooting began just before 6:30 p.m. Monday when Shane Tamura got out of his double-parked sedan and walked across a plaza to the entrance of the 600-foot-tall skyscraper that houses the NFL and investment firm Blackstone, according to police.

The building had a lengthy check-in process, security officers and locked turnstile gates leading to the elevators, CNN sports anchor Coy Wire said. “It is a massive building, one of the most highly secured that I’ve ever been in,” he added.

Wearing sunglasses and holding an M4 assault-style rifle, Tamura entered the building’s lobby and fatally shot uniformed NYPD officer Didarul Islam in the back, police said. Surveillance footage showed he then shot a woman behind a pillar and sprayed the area with gunfire.

At the office entrance, glass panels and a revolving door were shattered, though the pieces remained in their frames.

The shooter then moved toward the gates leading to the elevators and fatally shot Aland Etienne, a building guard who was trying to set off an alarm behind his desk that would have deactivated the elevators.

In the lobby, employees trying to escape the chaos shattered the glass barriers leading to the elevator area, a law enforcement official briefed on the situation said.

The gunman reached the elevators as one opened into the lobby, letting out a woman. After allowing her to walk past unharmed, he took the same elevator to the 33rd floor. There, at the office of Rudin Management, he stepped out of the elevator, walked around firing, and fatally shot one woman. He then walked down a hallway and shot himself in the chest.

That floor had a safe room designed for protection from attacks, New York Mayor Eric Adams told CBS News.

Images of the gunman brazenly walking into the office building show he was determined to carry out the attack, regardless of the security measures in place, law enforcement experts said.

“This person got out of his car, double-parked in Manhattan, walked straight into the building with a rifle, not attempting to hide his identity at all,” said Jason Pack, a retired FBI supervisory special agent. “So that tells you right there, he had no escape plan, he wasn’t worried about someone seeing him.”

The ‘Four Ds’ of physical security

In general, companies conduct a risk and vulnerability assessment to determine how much security they consider necessary, ranging from none to Fort Knox, Wheatley said.

The assessment aims to balance the risk of a trespasser with the need for employees and customers to move through the building efficiently, as well as considerations for cost, customer service, brand image, and other factors.

“Once you do those assessments, then that gives you some direction as to the levels of security you may need to put in place,” Wheatley said.

Occasionally, in shootings like this, companies look for the “shiny new toy” in security technology, said Sal Lifrieri, president of the security firm Protective Countermeasures.

“When you’re conducting the assessments, there is a lot that goes into it with respect to, OK, what’s the real threat?” he said. “What are we trying to stop? What do we need to be able to stop that? And then how does that go along with maybe the company or the organization’s goals? And then how much money do they have to spend, because they may not be able to afford it, right?”

Security experts refer to physical security efforts using what’s called the “Four Ds”: Deter, detect, delay and detain. In short, security layers should deter someone from attempting an attack, detect an incoming threat with surveillance, delay the attack by limiting access, and detain the individual, according to Wheatley.

There are also proactive efforts to prevent workplace violence, such as monitoring threats, training staff on warning signs, and establishing guidelines for threat management, he said.

CNN chief law enforcement and intelligence analyst John Miller noted the existence of “layered security” in the Park Avenue building.

“There is no layer of security for a regular public building, where thousands of people come in the course of several hours every day, that is going to address somebody who walks in with a fully loaded assault weapon, additional magazines and ammunition, who is bent on killing and opens fire the minute he walks in,” he said.

“That’s not a security issue, that’s an American issue about where we are now with people, their issues, guns, and how they deal with them.”

CNN law enforcement analyst Jonathan Wackrow said corporate security in NYC is “the best in the world” since the terrorist attacks of September 11, 2001, and buildings are “fortified to the Nth degree.”

“Right now, this is a seminal moment for corporate security programs here in Manhattan to go back and reassess because the threat landscape has now changed, we know this was a successful attack,” he said. “Property owners need to now go back and make sure their programs are aligned to this new threat environment.”

Employees take their own security steps

While the security infrastructure is vital during any attack, the response of employees inside the building is equally important.

“(Security) is a team sport,” Wheatley said. “It requires everyone; it’s not just the security department or the security personnel’s singular responsibility.”

In Monday’s shooting, people in the building and nearby appeared to follow the key “run, hide, fight” strategy often taught by security experts.

Videos from the scene show employees running out of the building with their hands raised. Iris Christo Doulou, who watched from her office across East 52nd Street, told CNN she saw people running outside and soon noticed a helicopter hovering overhead. She and her colleagues were told to stay inside.

Photos posted to X by an employee show workers inside the 345 Park Avenue building pushing furniture against their office entryway to create a barricade during the attack. The photos show couches and office chairs stacked against the door, nearly reaching the ceiling.

It is unclear which floor the photos were taken on.

Jessica Chen, an employee in the building, similarly told ABC News they ran into a conference room and “barricaded the tables across the doors and just stayed still.”

“A lot of us were young, a lot of us went through training in elementary school on what to do in an active shooter situation. We were all unfortunately prepared,” she said.

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CNN’s Jeff Winter, Gordon Ebanks, Andy Rose, Rachel Clarke, Elise Hammond, Sharif Paget, Taylor Galgano and Martin Goillandeau contributed to this report.