Paris, Oct 14 : French Prime Minister Elisabeth Borne has placed the country on under the highest security alert after an assailant fatally stabbed a teacher and injured three other people at a school in the northern city of Arras. The attack occurred at around 11 a.m. on Friday in Gambetta High School, reports Xinhua news agency. The police have identified the attacker as 20-year-old Russian national Mohamed Mogouchkovof Chechen origin. He is currently under police custody. "In the current context and following the terrorist attack in Arras, I have decided to raise the security (preparedness) to attack emergency level," Borne posted on social media late Friday night. Speaking to the French public television TF1, Minister of Interior Gerald Darmanin said that the government will mobilise "a few thousand more" numbers of the security forces to monitor places where the public gathers. Concerning the suspect who carried out the knife attack and is now in police custody, Darmanin said the perpetrator had been flagged by French security services for radicalisation. "We suspected something," the minister told TF1, adding that the man's phone had been tapped for several days prior to the incident, but there was no indication of a planned attack. Jean-Francois Ricard, public prosecutor covering anti-terrorism cases, told a press conference on Friday that the assailant is a former student of the school. The attacker first stabbed the teacher who was in front of the school, then another person who came to help the victim, Ricard said. Then after entering the building, the perpetrator injured two more people, both working in the school. Witnesses say the assailant shouted "Allahu Akbar", or "God is greatest", during the attack. Following the attack, President Emmanuel Macron visited the school and condemned the "barbarity of Islamist terrorism". He called on French people to stay "united" in the face of the attack, to "not give in to terror or let anything divide us", the BBC reported. Friday's he attack came nearly three years since the murder and beheading of another teacher, Samuel Paty, at his school outside Paris. The perpetrator of that attack, 18-year-old Abdullakh Anzorov, a Russian Muslim refugee, was shot dead by police. France has been hit by a series of Islamist attacks in recent years. The worst was in November 2015 when gunmen and suicide bombers attacked entertainment venues and cafes in Paris, killing 130 people. The attack was claimed by the Islamic State (IS) terrorist group. /IANS