A major tech sell-off stretched into a second day Tuesday as investors questioned whether artificial intelligence will generate the profits that have fueled lofty valuations for companies such as Alphabet, SpaceX and Nvidia.

After the opening bell, the tech-heavy Nasdaq Composite slumped 628 points, or 2.4%, to 25,537. The decline marks a second day of losses, with the Nasdaq closing 1.3% lower on Monday.

The Dow Jones Industrial Average slipped 305 points, or 0.6%, to 51,407 while the S&P 500 tumbled 1.6%.

For months, Wall Street has embraced tech stocks, helping drive the market to record highs on optimism that companies pouring billions into AI would translate those investments into faster revenue growth and higher profits. Investors are now demanding more evidence that the spending will pay off.

On Tuesday, several of the stocks that have powered the market's rally came under pressure as skepticism grew.

"For a long time, the market treated AI spending as unquestionably positive," Nigel Green, CEO of the financial consultancy deVere Group, said in an email Tuesday. "Investors are now becoming more demanding. They want evidence that unprecedented spending will translate into unprecedented profits."

Major tech stocks fell sharply. SpaceX shares dropped $4.09, or 2.7%, to $150.51 after plunging 16% Monday. Earlier this month, investors piled into the stock after the company's initial public offering, sending it above $200 within days.

The stock, however, has slumped for four consecutive trading days as investors fret over whether the company can justify its valuation of more than $2 trillion.

Bret Kenwell, a U.S. investment and options Analyst at eToro, told CBS News that a broader weakness and global volatility in tech stocks is weighing on U.S. shares. On Tuesday, chip-maker Nvidia fell 3.4%, while Broadcom tumbled 2.4%.

Alphabet, part of the Magnificent Seven, also came under pressure in early trading, falling 1.2%.

While the stock sell-off ignited sharp declines, Green said he doesn't believe markets are in trouble.

"What we're witnessing now is investors demanding proof instead of promises," he said. "That shift can be uncomfortable, but it's ultimately healthy."