U.S. stocks shuddered on Tuesday as earnings reports from two industrial giants at the heart of the economy flashed warning signs.

On one of the busiest earnings days of the year, heavy equipment manufacturer Caterpillar told shareholders its business growth could slow and warned its dealers worldwide that it would raise prices because of the higher cost of steel in the wake of U.S. tariffs.

3M, a global conglomerate, caught many by surprise by reporting slowing sales growth and lowering its outlook for the year.

Investor unease is running high as the historic bull market — which has pumped out trillions of dollars in returns over a decade — has signaled vulnerability in recent days.

Markets are getting hit from many sides: fears that the Chinese economy is slowing; uncertainty over an upcoming U.S. election; rising interest rates in the United States; and worries that the long bull market is running out of gas.

U.S. markets opened in a steep slide on one of the busiest earnings days of the year. (Brendan Mcdermid/Reuters)

By close of trading Tuesday, all three indexes had pared significant early losses. The Dow Jones industrial average, which was off 548 points at its low, closed the day down 126 points, or 0.5 percent, to land at 25,191.43.

The Standard & Poor’s 500-stock index lost 0.6 percent, its fourth decline in as many days. The tech-heavy Nasdaq, which has been whacked in recent weeks from the sell-off in the so-called FAANG shares — Facebook, Amazon, Apple, Netflix and Alphabet’s Google — dropped 0.4 percent.

All three had slipped to three-month lows in volatile morning trading.

The deepening tensions between the United States and Saudi Arabia after the death of journalist Jamal Khashoggi also weigh heavily on Wall Street. Saudi Arabia’s ability to temper oil prices makes it a huge player in the global economy.

Closely followed Brent crude dropped more than 4 percent to $76 per barrel, a key threshold that signals a plentiful supply, at least for the near term.

“Politics and earnings,” said Sam Stovall, chief investment strategist of U.S. equity strategy at CFRA. “Both 3M and Caterpillar appear to be spinning their wheels and showing the true effects of global tensions.”

Some analysts said the markets overreacted.

“This is the first time in several quarters Caterpillar hasn’t raised estimates,” analyst Rob Wertheimer of Melius Research said. “So it didn’t surprise as positively as in recent quarters. It was not a debacle.”

Another factor roiling the markets is the Federal Reserve’s commitment to gradually raise interest rates, which has drawn criticism from President Trump. Presidents historically prefer low interest rates because they help boost the economy and, in turn, help the popularity of whoever is in the Oval Office.

Rate increases have raised the U.S. 10-year Treasury bond to around 3.2 percent, its highest rate in years. The 10-year is closely followed because it is another indicator of the economy and the future of the stock market. Higher interest rates on the 10-year could steer investors to sell equities and turn to the less-risky Treasury note.

“Here is what is driving the markets the last 10 days,” said Scott Wren, senior global equity strategist at Wells Fargo Investment Institute. “Is the Fed going to make a mistake? Is global growth going to slow down? What are earnings going to be in 2019? We don’t think global growth is going to slow. We don’t think the Fed is going to make a mistake. And we don’t think there is going to be an all-out trade war.”

Oil prices slid Tuesday after Saudi Arabia said it would increase production, which would keep world supply and demand for oil in balance. Oil prices are based on a careful choreography between producers and consumers, with politics, economic growth, weather, accidents, terrorism and a million other factors affecting oil prices.

West Texas Intermediate futures were also trading nearly 5 percent lower at $66 per barrel.

A Saudi surge would make up for any Iranian shortfall that accompanies the economic sanctions that are going into effect next month against that country. President Trump withdrew from the nuclear agreement with Iran earlier this year, removing Iran supply from most oil markets.

“You look at everything going on — Saudi Arabia, the oil market, Brexit, Italy, the U.S. elections, the Fed raising rates — I could go on and on,” said Brad McMillan, chief investment officer for Commonwealth Financial Network. “The question isn’t why is the market reacting? The question is: Why isn’t it worse?”

McMillan said he sees the market dropping even more as part of a reality check on the economy and global tensions. The earnings season so far has been healthy, although some companies have reported a slowdown in revenue growth.

Analyst Matt Arnold of Edward Jones points to positive signs.

“Looking across companies, there are still healthy demand levels,” he said. “Costs are going up. Labor inflation is real. In periods of economic strength, that is what happens.”

But markets have seen several pullbacks, including an 11 percent decline in late 2015 and early 2016. Then the market dropped nearly 10 percent earlier this year.

“This is normal volatility. I would not be surprised if there’s more of a pullback,” McMillan said. “But as long as economic fundamentals remain solid, the market usually comes back.”