Stocks tumbled on Wall Street Friday after the U.S. announced plans to expand its trade war to Mexico, its third-biggest trading partner.

The slump all but guarantees the market will end May with its first monthly loss of 2019. The S&P 500 index was on track for its fourth straight weekly decline. The benchmark index has lost about 6.5% this month.

Bond prices rose again, sending yields lower. Oil and gas prices fell sharply.

The new front in the trade war is hitting automakers particularly hard. Many of them import vehicles into the U.S. from Mexico. General Motors and Fiat Chrysler were each down at least 4%.

Technology stocks suffered some of the heaviest losses. Banks also declined as higher bond prices pushed yields lower. Energy companies sank following another broad slide in oil prices.

Investors have been fleeing to safer holdings all month.

Copyright 2019 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.