Germany has updated its travel advisory for the United States to emphasize that a visa or waiver allowing entry does not guarantee Germans to enter the U.S. after several of its citizens were detained at the border.

According to an advisory on Germany’s Foreign Office website, the country warned that entry through the Electronic System for Travel Authorization (ESTA) system or a U.S. visa is not entitled every time and that the U.S. border control has final decision.

“It is recommended that you bring proof of your return journey upon entry,” the office said.

The office noted that a criminal conviction in the U.S., false information about an individual’s purpose of stay, or “even a slight overstay of the visa” can lead to arrest, detention and deportation. 

The new advisory comes as Germany is investigating the case of three of its citizens being denied entry and placed in detention when they tried to enter the U.S.

A spokesperson said “the incidents of the past week involving German citizens entering the USA” are being taken very seriously, local media reported.

Most tourists from European countries, including Germany, can generally travel to the U.S. for up to 90 days. Still, when the German citizens were detained separately crossing the San Diego-Tijuana border, they were denied entry and sent to a detention center, The New York Times reported.

The changes and detentions come amid the Trump administration’s crackdown on immigration at the country’s southern border and the implementation of its mass deportation plan.

While President Trump campaigned to stop illegal immigration, he’s primarily targeted long-standing programs that give migrants and travelers legal ways to enter the U.S.