A gunman killed at least four people and wounded 11 others near a Christmas market in the French border city of Strasbourg on Tuesday evening before being cornered by police.

The motive was not immediately clear but, with France still on high alert after a wave of attacks commissioned or inspired by Islamic State militants since early 2015, the counter-terrorism prosecutor opened an investigation.

Some two hours after the attack, elite police cornered the suspect and shots were fired, a source close to the operation said. French media reported the assailant was holed up in a store on the Rue Epinal.

Interior Minister Christophe Castaner said the gunman was known to security services, and the local prefecture said he was on an intelligence services watchlist.

Castaner said France was raising the security threat level and would bolster the protection of Christmas markets elsewhere and strengthen border controls.

People in the city’s Neudorf area and Etoile park were told to stay where they were as officers hunted the shooter on the ground and from the air.