The Standard & Poor’s 500 index closed Friday’s session down 2.6% on the week and recorded its first monthly drop since December as worries about trade tensions increased amid new threats by the US on Mexico.

The index ended the week at 2,752.06, down from last week’s closing level of 2,826.06 and marking its largest weekly drop since the week of Dec. 17 through 21, 2018. Friday’s closing level also marks a 6.6% drop for the month of May.

The index has now been down for four consecutive weeks, an event not seen since October 2014, according to Howard Silverblatt, senior index analyst at S&P Dow Jones Indices.

However, the measure is still up 9.8% for the year to date as May’s drop was outweighed by the index’s gains in the first four months of the year.

Every sector of the S&P 500 fell this week, led by the energy sector, which tumbled 4.4% as crude-oil futures fell amid threats by US President Donald Trump to impose increasing tariffs on Mexico, effective June 10.

The US tariffs on Mexican imports would begin at 5% and reach 25% on Oct. 1 unless “the illegal migration crisis is alleviated through effective actions taken by Mexico, to be determined in our sole discretion and judgment,” which would result in the removal of the tariffs, Trump said.

The worries about trade issues with Mexico come at a time when investors have already been concerned about the potential economic impact of the US’s trade dispute with China.

Among the energy sector’s decliners this week, shares of Cimarex Energy (XEC) slid 11% and Marathon Oil (MRO) dropped 8.9%. Marathon Oil said it completed the sale of its 15% stake in the Atrush Block in Kurdistan, officially marking the company’s exit from Iraq.

The consumer-staples sector had the next-largest percent drop off the week, falling 3.5%.

Kraft Heinz (KHC) was among the consumer-staples sector’s hardest-hit stocks this week, sliding 11%, as Credit Suisse cut its price target on the stock, citing the possibility that a Securities & Exchange Commission investigation into the company “will continue to reveal breakdowns in the company’s internal controls and negatively impact the company’s forward outlook.”

Also in the consumer-staples sector, shares of General Mills (GIS) fell 6.4% as Goldman Sachs downgraded its investment rating on the food-products company’s shares to sell from neutral.

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