(LifeZette) Arrests along the U.S.-Mexico border spiked 26.5 percent from April to May, the first monthly increase since Donald Trump became president, according to statistics released Friday.

Illegal Border Crossings Spiked 27 Percent in May

U.S. Border Patrol agents caught 13,535 people at the border last month, while authorities stopped another 5,432 ineligible people who tried to come into the country at an entry point. The combined total of 19,967 people was an increase over the 15,780 foreigners who tried to cross the southern border the previous month, according to U.S. Customs and Border Protection (CBP) statistics.

Experts generally estimate that for every foreigner prevented from making an illegal crossing, one gets through. That suggests that illegal immigration increased last month.

Jessica Vaughan, director of policy studies at the Center for Immigration Studies, said one month’s worth of data is insufficient to draw sweeping conclusions.

“It shows the hazards of reacting on a monthly basis,” she said. “There are going to be ups and downs. We need to look for trends.”

Experts have attributed the initial drop to people in Mexico and Central America as responses to the president’s tough rhetoric. But Vaughan said that alone will not work forever.

“It underlines the need for actual enforcement,” she said. “Messaging is not enough by itself.”

Joseph Guzzardi, a spokesman for Californians for Population Stabilization, said the increase likely is partially a natural blip due to warmer weather and partially the result of the fact that few of Trump’s policy proposals have taken effect.

“I would say to President Trump, you’d better do everything you can to make sure the internal enforcement remains,” he said.

Total apprehensions remain well under the figure for May 2016, when authorities apprehended 55,442 border crossers. That was the second-highest figure of any month in 2016. Illegal border crossings are still on pace to drop dramatically by the end of the fiscal year from the 408,870 recorded in fiscal year 2016.

But Vaughan said sustained progress will require more than talk. She said smugglers constantly change their methods to evade enforcement efforts.

“As long as people believe a job will be waiting for them here and they will get away with being here, they are going to pay that money to smugglers,” she said.