ARRAS, France -- A man of Chechen origin who was under surveillance by French security services over suspected Islamic radicalization stabbed a teacher to death at his former high school and wounded three other people Friday in northern France, authorities said.

France raised its threat alert to its highest level, and the attack was being investigated by anti-terrorism prosecutors amid soaring global tensions over the war between Israel and Hamas. It also happened almost three years after another teacher, Samuel Paty, was beheaded by a radicalized Chechen near a Paris area school.

The suspected attacker had been under surveillance since the summer on suspicion of Islamic radicalization, French intelligence services told The Associated Press. He was detained Thursday for questioning based on the monitoring of his phone calls in recent days, but investigators found no sign that he was preparing an attack, Interior Minister Gerald Darmanin said.

’’There was a race against the clock. But there was no threat, no weapon, no indication. We did our our job seriously,″ Darmanin said on TF1 television. French intelligence suggested a link between the war in the Middle East and the suspect’s decision to attack, the minister said.

The suspect, identified by prosecutors as Mohamed M., was reportedly refusing to speak to investigators. Several others also were in custody Friday, national counterterrorism prosecutor Jean-Francois Ricard said. Police said the suspect’s younger brother was among those held for questioning.