The suspect in a deadly shooting spree at a Jewish school in France knows he is being hunted and may carry out another attack, chief Paris prosecutor Francois Molins warned Tuesday.
He said the killer is "very determined" and has committed premeditated murders, targeting victims who had been selected according to their race or religion.
"The criminal is anti-Semitic or terrorist," Molins said. "One does not exclude the other."
The shooting on Monday was the third fatal attack on people from minority groups in southwest France in the space of eight days.
The region remains on scarlet alert, the highest possible level, after a teacher and three children -- two of them his own young sons -- were gunned down at the Ozar Hatorah school in Toulouse on Monday. The other victim, the daughter of the school's director, was killed in front of her father.
The other two incidents targeted soldiers of north African and Caribbean origin.
Molins said it was clear the same weapon had been used in all three attacks.
Exceptional measures have been put in place to find the suspect as soon as possible, Molins told reporters at a news conference in Paris. Investigators must verify witnesses' accounts and analyze some 7,800 hours of surveillance footage, he said.
Witness accounts indicate that the criminal is slim and around 1.75 meters tall (5 foot 9 inches), he said, although inquiries are still in progress.
All the victims were shot in the head at point blank range, he added.
Interior Minister Claude Gueant earlier said authorities are investigating the possibility that neo-Nazis were behind the attacks.
The gunman wore a camera on his chest during the attack, Gueant told Europe 1 in an interview Tuesday.
The minister said a witness had told authorities about the device, but Gueant did not know if the gunman had recorded the attack.
France observed a minute of silence in memory of the victims on Tuesday morning, with President Nicolas Sarkozy marking it at a school in Paris.
"This happened in Toulouse, in a religious school, to children from Jewish families, but it could have happened here. It could have been the same assassin. These children are exactly like you," he told the pupils.
Fear, confusion on streets of Toulouse
The bodies of the four victims have arrived at Paris-Charles de Gaulle Airport, said an official with a group representing Jewish communities in France, the Consistory of Paris.
Sarkzoy and Joel Mergui, the president of the Consistory of Paris, were present to receive them.
The bodies will be flown to Israel by the El Al airline late Tuesday night, the Consistory said. Their burial is expected to take place Wednesday morning in Jerusalem.
Sarkozy wrote to Israeli Prime Minister Benjamin Netanyahu on Monday to express his condolences for the loss of the victims, three of whom -- the teacher and his sons -- held dual Israeli-French citizenship.
The teacher was born and raised in Bordeaux, in southwestern France, but went to Israel to pursue his religious studies. He married and had a family before returning to teach at the Toulouse school, the Consistory said.
The decision to send the victims' bodies to Israel is because of their faith rather than their nationality, the Consistory said, since as practicing Jews, their burial in the birthplace of Judaism ensures that their remains will not be tampered with. Forty percent of French practicing Jews are buried in Israel, according to the Consistory.
Sarkozy, who is running for re-election, has suspended his campaign in light of the wave of violence against minorities.
One of the guns used Monday was also used in the killings of French soldiers of north African and Caribbean origin on March 11 and March 15, said Elisabeth Allannic, a spokeswoman for judicial authorities in Paris.
The first victim was a 39-year-old man, shot in Toulouse, while the other two were ages 24 and 26, according to French authorities.
A court in Paris has opened an investigation into all three killings, under anti-terrorism powers.
It is the first time a scarlet alert has been declared in France, French media reports say. The status means the state can implement sweeping security measures to guard against an imminent threat of major terror attacks.
Measures include increased security at schools, extra surveillance of Jewish and Muslim sites, restrictions on traffic and access to public buildings, additional police on duty and extra scrutiny of passengers and baggage on publis transport, the local Haute-Garonne authorities said in an online statement Monday.
France has a complex history with the far right.
There has been steady if minority support for the National Front party founded by Jean-Marie Le Pen and now led by his daughter Marine. The senior Le Pen came in second in presidential elections in 2002, and his daughter is a candidate this year.
Sarkozy himself said in an interview earlier this month that France has too many immigrants.
"Our system of integration is working worse and worse, because we have too many foreigners on our territory and we can no longer manage to find them accommodation, a job, a school," he said in an interview on France 2 TV on March 6.
Historian and author Patrick Weil told CNN it is likely the public's shock over the attacks will lead to a calming down of campaign rhetoric directed at minorities.
The Israeli government earlier said the bodies of the four victims of the school shooting would be flown to Israel for burial.
"The government of Israel has decided to transfer the coffins to Israel as soon as possible, with the cooperation and assistance of Israel's representatives in France," the Israeli foreign ministry said in a statement.
The gunman pulled up in front of the Jewish school just before 8 a.m. on Monday and started shooting, authorities said. He got close enough to shoot his victims in the head, local journalist Gil Bousquet said.
The gunman wore a motorcycle helmet and fled on a motor scooter after the shootings, interior ministry spokesman Pierre-Henry Brandet said. The same method was used in the earlier soldiers' shootings.
France, which has one of the largest Jewish populations in Europe, had 389 reported acts of anti-Semitism in 2011, according to the Representative Council of Jewish Institutions in France, known by the French acronym CRIF.
The group later issued a statement saying that, while it is too early to determine definitively the motive for the "horrible" crime, it appears to be a case of anti-Semitism. It called for increased security at places of worship and study as the investigation continues.
Security heightened in U.S. synagogues
Police in New York, Washington and San Francisco all said they plan to increase patrols of synagogues and Jewish institutions, with New York Police Commissioner Ray Kelly citing fears someone might stage a "copy-cat" attack. But he and the city's mayor stressed there is no "specific" intelligence indicating an active threat there.
"Just because there's something that happens there, doesn't mean there are more threats here. But we take everything very seriously," said Mayor Michael Bloomberg.