President Trump on Wednesday sought to stoke more concern about migrant caravans slowly making their way toward the United States, highlighting a violent clash with Mexican federal police that took place earlier this week at the Guatemala-Mexico border.

“The Caravans are made up of some very tough fighters and people,” Trump wrote on Twitter. “Fought back hard and viciously against Mexico at Northern Border before breaking through. Mexican soldiers hurt, were unable, or unwilling to stop Caravan. Should stop them before they reach our Border, but won’t!”

The episode cited by Trump took place Sunday as a caravan of roughly 3,000 Central American immigrants crossed onto Mexican soil, with some throwing rocks and bottles. One migrant was killed, and dozens were injured.

In the run-up to next week’s midterm elections, Trump has repeatedly sought to focus attention on migrant caravans, using them as a symbol of problems with the U.S. immigration system and blaming Democrats for a failure to pass legislation in the Republican-led Congress.

The caravan mentioned in his tweet is lagging behind another caravan that earlier crossed into Mexico and is making the long journey toward the United States by foot. Most of those traveling in the caravans are mothers and children, many with hopes of seeking asylum in the United States.

In a separate tweet Wednesday, Trump asserted that the caravans include “some very bad thugs and gang members” and urged them to “TURN AROUND!” He referred to his order to increase the number of troops deployed to the border to assist border agents.

john.wagner@washpost.com Nick Miroff contributed to this report.