Minnesota exports to other countries fell in the first four months of this year as they did across the Upper Midwest, new data from the Federal Reserve Bank of Minneapolis show.

Minnesota set a record in exports last year even as a trade war broke out between the U.S. and some of its biggest trading partners, including China, Canada, Mexico and the European Union.

The state was one of three among the six in which the Minneapolis Fed oversees banking and monitors economic performance, to have exports rise last year. But exports fell in all six states during the first four months of 2019, the regional bank said in a roundup of new data.

North Dakota and South Dakota were hit hardest. Minnesota was hit the least, with just a 1% drop.

Fed researchers said it's difficult to show cause and effect but they note that the countries that have imposed retaliatory tariffs on the U.S. are some of the biggest markets for Midwestern states. In comparison, declines also occurred in the early months of 2018 before President Donald Trump last May imposed the tariffs on steel and other metals that led to retaliation from other countries.

"In each market embroiled in a trade war, exports targeted by retaliatory tariffs generally saw bigger decreases than other exports," Tu-uyen Tran, an economic researcher at the Minneapolis Fed, wrote in a report for the bank's Fed Gazette publication.

Tran noted several flaws in the data used in the report, which came from the U.S. Commerce Department. Agricultural products, for instance, are intermixed at storage facilities before being put on cargo ships and officials typically list the export site as the port rather than the state from which products came.

There's no way to get a complete picture on ag products from the data, as a result. But the data collection remains consistent, meaning the year-to-year comparisons have some value.

Minnesota's exports grew 7% in 2018, according to the Commerce data. That is slightly less than the 10% growth reported by state economic-development officials earlier this year. For the first four months of this year, Minnesota's exports fell 1%, the Commerce data showed.

State officials will give their first report on 2019 exports in September, when they report six-month performance.

Among the other states in the Minneapolis Fed region, North Dakota saw the biggest reversal in shipments. Its exports rose 7% in 2018 but were off 14% during the first four months of this year.

In South Dakota, exports rose 3% in 2018 but were down 12% at the start of this year. In Wisconsin, exports fell 1% last year and were down 4% in early 2019.

In Montana and Michigan, export declines were slower at the start of this year than in 2018.

In Montana, exports were down 4% in 2018 and 3% in the first four months of this year. In Michigan, the decline was 5% last year and 4% at the start of this year.