Barriers were erected in front of the ritzy Midtown Manhattan office building where a crazed gunman killed four people Monday, with access being restricted even to some who work there.

“I’m a contractor, I work in the building. My tools are in there, I came today to get them, they told me it’s an active crime scene. I can’t go,” a worker who was turned away at the entrance told The Post Tuesday.

Even before the horrific rampage, the 44-story skyscraper was “highly secured” and featured a multi-step process both for entry and elevator access, according to a former NFL player.

“It is a massive building, one of the most highly secured that I’ve ever been in,” CNN sports anchor and ex-NFL player Coy Wire told the outlet, describing the lengthy check-in process required for entry and locked security gates restricting access to the elevators.

But Shane Tamura, 27, of Las Vegas, somehow gained entry to 345 Park Ave. — which houses Blackstone, NFL Headquarters, Rudin Management and other businesses — armed with an AR-style assault rifle.

Wire said friends who work in the building were “scared” and “shocked” even before they knew an NFL employee had been shot.

Tamura had planned on targeting employees of the NFL, implying he was suffering from CTE from playing in the league, even though “he never played in the NFL,” Mayor Eric Adams told Fox 5 Tuesday.

In the wake of the assassination of UnitedHealthcare CEO Brian Thompson in Midtown last December, dozens of security chiefs at major US firms met to discuss whether they should beef up security protocols to protect top executives.

Investment bank Blackstone, which had offices at 345 Park Avenue where the shooting took place, spent $4.6 million on after-hours security measures for principals in 2023 alone, according to Equilar.

Tenants occupying buildings close to the site of Monday’s killings are already asking to step up security measures, a man who works at a commercial building on East 51st Street who declined to give his name told The Post.

“We’re seeing more people asking for extra training on security and the emergency action plan — what they’d do in a situation like what happened yesterday,” he said, noting such trainings usually take place once a year.

“But they’re asking for more than that. One tenant is asking for another training as early as next week,” he said.

“People are scared. We have security guards but they don’t have guns. You need an access card to get up to the different floors. You do have to swipe at the elevator, it will stop at that direct floor, it can’t go to any other floor.”

He said he thought it would be a good idea to have armed security guards in every building so “at least you have something” in the event of a mass shooting.

Tamura began his massacre in the lobby of the 44-story building, shooting and killing police officer Didarul Islam, 36, as well as married mom and Blackstone executive Wesley LePatner, who was hiding behind a pillar.

He unleashed a barrage of bullets on his way to the elevators, killing a security guard who was crouching at his desk.

Tamura then rode an elevator to the 33rd floor and opened fire as he stalked the office, killing Julia Hyman of Rudin Management before turning the gun on himself.

The deranged man wanted to shoot up the NFL’s offices on floors 5 through 8, but “mistakenly went up the wrong elevator banks” and ended up on a higher floor,” Adams confirmed.

Bystanders near the building described hearing shots emanating from the skyscraper lobby, and spotting Tamura toting a “big ass gun” as they swiftly walked out of the gunman’s path.

How the shooting unfolded

• Reports of the shooting at 345 Park Ave. start coming in around 6:28 p.m.
• Shane Tamura, 27, is seen getting out of a black BMW between 51st and 52nd streets with an M4 rifle.
• He enters the lobby and turns right, where he shoots police officer Didarul Islam, 36, dead.
• Tamura guns down a woman cowering behind a pillar in the lobby, sprays more bullets and walks toward the elevator bank — where he shoots dead a security guard crouching at his desk.
• One more man reports being shot and injured in the lobby. He was in critical but stable condition.
• The gunman allows a woman to walk out of the elevators unharmed before heading up to the 33rd floor, where building owner Rudin Properties’ offices are located, “and begins to walk the floor, firing as he traveled.”
• One woman is shot and killed on that floor before Tamura shoots himself in the chest.
• It’s unclear how long the mayhem lasted. Police Commissioner Jessica Tisch posted on X at 7:52 p.m.: “The scene has been contained and the lone shooter has been neutralized.”

“I heard a gunshot. I wasn’t really looking at his face, but I saw the gun. He was holding it like this,” AB, 17, who works at Taste of Delicatessen on Lexington Ave., told The Post, standing with his feet apart while mimicking holding a long gun.

“It was a big ass gun. I didn’t want to be in the danger zone so I walked out of the way and left it alone,” he said, describing people running for their lives amid the chaotic scene.

“It was crazy.”

He said he wasn’t nervous because he lives in Red Hook and has heard gunshots there — but not in Midtown in broad daylight.

“God was with me, so if he told me to walk out of the way, I did it. It is what it is.”

Jason Matza, 35, a stock trader, said he lives in the area and passes 345 Park Ave. on a regular basis on his way to and from work.

“I was gonna walk by here last night before I heard about the shooting. I could have been right here,” he told The Post.

Matza’s father was in town to see his new apartment when he got an alert on the Citizen app about the shooting in progress.

“I was like holy s–t. You got Grand Central literally down the street so he could’ve had access to any form of transportation. It could’ve been an uncontained situation,” he said.

“I’m glad it wasn’t a terrorist attack.”

Griffin Harblin, 22, an intern at a building across the street from the scene of the mass killing, said it was “a bit scary” but praised the NYPD, which he said did “a good job” handling the incident.

“Obviously I feel for the families and the people affected by this, but obviously it could have been a lot worse. So yeah, I’m feeling very thankful on this Tuesday morning.”