A shifting narrative about when officials at Mandalay Bay Resort & Casino alerted cops before the worst mass shooting in US history has Las Vegas on edge. And now gamblers are wondering: are they rolling the dice with their own lives inside the city’s casinos?

Safety — security experts told The Post — is no sure thing.

“[Bad] things do go on,” according to a security executive who worked eight years at a major casino on the Strip. “Everything short of murder happens in these hotels every day . . . suicide, domestic violence, prostitution, drugs and cheating.”

He continued: “Few casinos have advanced active shooter training for their security teams.”

The mass shooting at Mandalay Bay took a turn not even the most trained security personnel could have anticipated. “Nobody thought of [a killer] breaking out the windows” to shoot at crowds 32 stories below, the executive said.

Clark County Sheriff Joe Lombardo said earlier last week that Stephen Paddock shot at Mandalay security guard Jesus Campos at 9:59 p.m. on Oct. 1 — six minutes before the attack. Hotel brass clarified last Thursday that there was a paperwork snafu, saying Campos was actually shot about 40 seconds before the bloodbath.

It is legal to open-carry guns in Nevada, but most if not, all Vegas casinos have policies forbidding it.

Surprisingly, many casino security guards opt not to carry firearms. At the majority of these places, said the executive, “there is extensive testing, including a firearms test and a psychological test” required in order for employees to be armed; many don’t want to go through the rigors.

“It doesn’t pay [for employees] to expose themselves to the liability,” said the executive, who himself kept a 9 mm semiautomatic pistol tucked inside a waistband holster when he worked for the casino. Explaining that the salary bump for being armed is minuscule, he added that his department “had a small percentage carrying.”

The thing is, casino security guards are not there to protect guests.

“[They] have no actual power except for being a visual deterrent,” said a veteran security trainer with more than 20 years in the business. “Guards are not allowed to interfere with anything related to guests, as far as weapons are concerned . . . because hotels are afraid of getting sued. The No. 1 concern for casino security is the cash box as it moves around.”

Instead, guards are instructed to call police for most everything. But layers of protocol dictate that such calls never come directly from a security guard; rather, that person contacts a dispatcher who reaches out to authorities, adding a bit of a time lag.

Still, the veteran trainer said, it’s “usually not a long delay [between incident and arrival of Metro police] because Metro is always on the Strip.”

Not that most guests will see cops in the casinos. A former guard who worked for a major casino company said that protocol has police using discreet entrances, so as not to draw attention and hurt business: “Cops come in through the back. They have a special entrance and are there every night.”

Even so, there are a smaller offenses casinos deal with internally, such as pickpockets and purse-snatchings.

For those, they often use decoys to tempt thieves. The former guard recalled catching a crook “with a fake broken arm” by leaving cash within easy reach at the casino pool club.

“For petty prostitution, petty larceny, property damage, we evict the people,” said the executive. “We don’t want security to be tied up, waiting for the police to arrive.”

If you get caught card counting — which casinos work hard to deter — you’ll be kicked off the game by security, not police, since it’s against casino rules but not illegal. In some cases, particularly for repeat offenders, players can be banned from the casino’s premises. Venture back and police will be called.

A bonafide cheater — someone who, say, marks cards — gets treated differently: “He’ll be handcuffed, taken into a backroom, and we might start interrogating him while waiting for a gaming control officer,” said the former guard. “They’re all over the Strip and have police power.”

And that’s what seems to be the casinos’ biggest concern.

“If you’re on the FBI most wanted list, you can disappear in Vegas,” said the veteran trainer. “ If you’re a card cheat, you’re on their radar. It’s about money.

Additional reporting by Cindi Reed