For the first time in French history, the highest terror-alert level was activated as rifle-carrying officers stood guard in city streets in southwest France, road checks were set up and a massive manhunt was on to catch the mysterious gunman who has killed three paratroopers, a rabbi and three Jewish schoolchildren.
In one of the most difficult tests for police, detectives are scrambling to catch a serial killer who could strike at any time, who has no visible ties to the victims and has left no manifesto or letter.
"We're facing an extremely determined person, who knows he's being hunted, who could attack again," chief prosecutor François Molins told reporters Tuesday.
The scooter-riding killer, who has struck three times at four-day intervals, may have filmed his last rampage at a Jewish school with a camera strapped to his chest.
While officials say they are checking whether the gunman had a support network, he appears to be of the hardest breed to track: the solitary operator who has not crossed paths with the law before.
"The lone wolf is practically impossible to detect until he acts again and makes a mistake," Jean-Yves Camus, an expert on extremist groups at the Institut de relations internationales et stratégiques, said in an interview from Paris.
More than 200 French investigators are now reviewing surveillance video footage, scouring Internet and cellphone records, looking for a stolen Yamaha motorbike, checking the whereabouts of former enlisted men and neo-Nazi suspects. Their actions focus on three main lines of investigation.
Unlike most homicide cases, serial killers have no apparent links to the victims. Investigators are looking for connections to earlier crime and trying to draw a profile by analyzing the shooter's possible motives.
"You've got a total stranger. That makes it real hard for the investigation," said Robert Keppel, a former Washington state detective who investigated the serial killer Ted Bundy.
The gunman attacked a Jewish school and soldiers of North American or Caribbean ancestry in the southwest cities of Toulouse and Montauban. He also appeared to have some training, considering his ability to handle firearms and his seemingly calm, focused behaviour during the attacks.
Police and military investigators have checked on army personnel, especially three members of the Montauban-based 17th Engineering Airborne Regiment who were disciplined in 2008 for neo-Nazi sympathy. There were reports that the three are no longer suspects in the shootings.
French Interior Minister Claude Guéant confirmed that the killer appeared to have a video camera on his chest during Monday's attack on the Ozar Hatorah school in Toulouse, though Mr. Molins cautioned it might have just been a strap.
"There are no neo-Nazi groups that I know of which would be capable of carrying out attacks with such professionalism and cold-bloodedness," Mr. Camus said.
The first victim, Staff Sergeant Imad Ibn-Ziaten, was shot March 11 as he was about to meet someone to sell his Suzuki Bandit motorcycle. He might have been in touch with the killer after posting an online classified ad, so police are likely trying to retrace that communication.
Mr. Molins said there are 7,800 hours of surveillance video to scrutinize. Police are also checking if cellphone calls were made around the time of each attack.
The gunman used the same .45-calibre pistol in all three shootings, adding a 9-mm submachine gun when he attacked the school. Since the .45-calibre is rare in France, investigators are looking at possible sources for the weapons and ammunition. Gun clubs and shooting ranges are being investigated.
The public front
Meanwhile, southwest France has been put under "scarlet" alert, the highest level of the country's anti-terror watch system.
Officers have been told to be on the lookout for a Yamaha T Max 530 scooter that was stolen five days before the first shooting.
Tips from the public have poured in. Le Figaro reported that police were tracking a female commuter in and around Paris after a tipster overheard her admonish someone on her cellphone, saying "Now, you're going after a school."
Mr. Camus was skeptical that the gunman would be caught soon but he noted that France's Jewish community has already been living under a state of heightened alertness for more than a decade, ever since the second intifada reignited tensions between Jews and Palestinians.