German police arrested three people in a car with a Belgian license plate near the border with Austria and are investigating whether they were planning an attack, a spokesman said.
The three suspects from Kosovo were arrested on March 22 before the attacks that killed at least 31 people at a metro station and the international airport in Brussels.
Police made the arrests on a motorway in the southern state of Bavaria after receiving an intelligence tip-off.
"Investigations have been launched into the suspected planning of a serious criminal act against the state because there was notification of that," the spokesman said.
He said there was no indication so far that the three suspects had any links to the attacks in the Belgian capital, but added that could not be ruled out.
Germany stepped up security measures at airports, train stations, and the borders with Belgium, France, the Netherlands, and Luxembourg after the explosions in Brussels.