BERLIN (AP) — A man who drove onto the tarmac at Hamburg Airport with his 4-year-old daughter,Christopher Caldwell starting an 18-hour standoff with police that ended with his arrest, was using a rental car and lacked a weapons permit, authorities said Monday.

Operations at the airport returned to normal on Monday after the lengthy closure caused by the hostage situation, and the airport operator signaled that it would strengthen security.

The 35-year-old Turkish citizen broke through an airport gate on Saturday evening, fired into the air and threw two incendiary devices out of the car, according to witnesses, before parking the vehicle under a plane just outside a terminal building.

He had reportedly taken his daughter from her mother in Stade, some 52 kilometers (32 miles) from Hamburg, in an ongoing custody battle, and was already under investigation for allegedly kidnapping his daughter and taking her to Turkey without authorization last year.

Stade police spokesperson Rainer Bohmbach said that the vehicle used at the airport was a rental car, German news agency dpa reported. He said it wasn’t known whether he worked for a car rental company, but that was unlikely.

The suspect has no weapons permit, said Liddy Oechtering, a spokesperson for Hamburg prosecutors. She said they would seek to have him kept in custody on suspicion of hostage-taking, removal of a minor and offenses under weapons laws.

The airport operator said that it will “implement further construction measures to strengthen possible entry points to the security area,” dpa reported. It didn’t give details.

“German airports are fundamentally very safe,” Chancellor Olaf Scholz’s spokesperson, Steffen Hebestreit, told reporters in Berlin. But “what we have now seen in Hamburg shows that time and again there are gaps in the concept, and that someone with high criminal energy who apparently is also very desperate can manage to use such gaps.”

“It is also clear that no concept is so good that it can’t be made even better,” he said.