There’s a serial killer on the loose — and the deadly enemy is volatility.

As the stock markets showed recently, volatility is back, with stomach-churning extreme highs and lows not seen since the Great Depression, according to analysts.

Volatility is the new market nemesis to the $27 trillion US retirement industry. And the sell-offs are contributing to fear in those nearing their golden years. “It certainly got people’s attention,” said Timothy Speiss at EisnerAmper Wealth Planning. “We’re going to have to see if the markets remain this choppy, with a lot of volatility.”

Some say signs already point in that direction. “With the market closing at the low end of the day’s range, expect more gyrations in the days and weeks to come,” said Greg McBride, chief financial analyst at Bankrate.com, looking at one recent session in which the Dow closed down more than 1,000 points.

Fed meddling, the rise of high-speed algorithmic traders and the introduction of exotic products like risk-parity funds that the average investor can barely comprehend have only intensified the present danger, seasoned pros are warning.

The volatility is a reminder of this dark new normal, and the threat to your 401(k) retirement savings.

“The fear of rising interest rates, a more aggressive Fed, automated speed traders and a possibly overheating economy led to the volatility,” financial adviser Edward Kohlhepp told The Post. Although additional bogeymen are blamed for the recent rout, analysts already detect a potentially disturbing pattern — the reappearance of over-the-top volatility.

Take just the week of Feb. 4. On Monday alone, the Dow lost almost 1,600 points, then quickly recovered some losses, only to close down 1,175.21 points, or 4.6 percent, to 24,345.75. On the following day, the Dow rebounded, posting a 567-point gain, and then Wednesday it closed down a mere 19 points. The following day, however, the bears were back. The Dow took another nauseating nosedive, plunging more than 1,000 points.

The market rout was now in a technical “correction,” meaning it had lost 10 percent as measured against the S&P 500 since Jan. 26.

“Those closer to retirement should not be aggressively invested,” Kohlhepp said. “They should have a plan that migrates them toward a more conservative, less risky, portfolio.”

The recent carnage just cast a spotlight on the emergent reality of extreme movements in volatility.

One researcher traces this to the decade that ended in 2000, when the S&P 500 had already expanded by a record 417 percent.

This extraordinary high was followed by three years of stock market declines of 10 percent, 13 percent and 23.4 percent, respectively. Three consecutive years of declines had only previously happened during the Great Depression and World War II, according to the researchers at Walser Wealth.

And since late 2008, the markets again have risen to new highs, which have in turn made the markets threateningly volatile.

“When the market declines sharply, everyone naturally wonders, ‘What’s wrong?’ ” one market pundit observed.

But some analysts have a worrisome answer: It’s the “natural” volatility.

And that could wreak brutal financial havoc.