An intense manhunt for two brothers wanted in the Charlie Hebdo magazine massacre focused Thursday on northern France's Picardy region, where sources close to the investigation said a police helicopter might have spotted the suspects.
Authorities believe that Cherif Kouachi, 32, and Said Kouachi, 34, entered a wooded area on foot, the sources told CNN's Chris Cuomo. Now investigators are using helicopters equipped with night vision tools to try to find them, the sources said.
Earlier Thursday, a police helicopter glimpsed what investigators believed to be the fugitives in the same area, near Crepy-en-Valois, France.
Police flooded the region, with heavily armed officers canvassing the countryside and forests in search of the killers. They came after a gas station attendant reportedly said the armed brothers threatened him near Villers-Cotterets in Picardy, stole gas and food, then drove off late Thursday morning.
About 10 kilometers (6 miles) from the gas station, police blocked a rural country road leading to the French village of Longpont. Authorities have not commented in any detail, but pictures showed heavily armed police officers with shields and helmets in the blocked-off area.
Hours later, a CNN team witnessed a convoy of 30 to 40 police vehicles leaving a site near Longpont.
As the search for the suspects intensified, details emerged about their past travels -- and possible training abroad.
The United States was given information from the French intelligence agency that Said Kouachi traveled to Yemen as late as 2011 on behalf of the al Qaeda affiliate there, a U.S. official told CNN. Once in Yemen, he received a variety of weapons training from al Qaeda in the Arabian Peninsula, including on how to fire weapons, the official said.
In addition to Picardy, other parts of France have also been under scrutiny.
Earlier Thursday, a gunman -- dressed in black and wearing what appeared to be a bulletproof vest, just like those who attacked the Charlie Hebdo offices -- shot and killed a female police officer in the Paris suburb of Montrouge. A municipal official was seriously wounded in that attack, France's interior minister said. One person was arrested, Paris Deputy Mayor Patrick Klugman said, though it's not known whether the shooter is still at large.
Authorities have called that a terror attack, but they haven't outright connected it to Wednesday's slaying of 12 at the satirical magazine's Paris headquarters.
Latest updates at 6:40 p.m. ET
• A U.S. law enforcement official told CNN that the Kouachi brothers were in the U.S. database of known or suspected international terrorists, known as TIDE, and also had been on the no-fly list for years.
• One of the brothers suspected in the Charlie Hebdo attack traveled to Yemen in 2005, France's justice minister told CNN on Thursday. Justice Minister Christiane Taubira did not specify which brother had traveled to Yemen.
• An ISIS radio broadcast Thursday praised the attackers, calling them "brave jihadists." There was no mention of a claim of responsibility for the attack.
• Paris' iconic Eiffel Tower went dark at 8 p.m. (2 p.m. ET) in remembrance of the victims of Wednesday's attack.
Charlie Hebdo to publish next Wednesday
While its business is satire, Charlie Hebdo has been the subject of serious venom.
That includes its publication of cartoons lampooning the Muslim prophet, Mohammed, which some found very offensive.
The magazine's offices were fire-bombed after that in 2011, on the same day the magazine was due to release an issue with a cover that appeared to poke fun at Islamic law.
It was attacked again Wednesday, when two masked men entered its offices not far from the famed Notre Dame Cathedral and the Place de la Bastille.
On their way into the building, they asked exactly where the offices were. The men reportedly spoke fluent French with no accent.
They barged in on the magazine's staff, while they were gathered for a lunchtime editorial meeting. The gunmen separated the men from the women and called out the names of cartoonists they intended to kill, said Dr. Gerald Kierzek, a physician who treated wounded patients and spoke with survivors.
The shooting was not a random spray of bullets, but more of a precision execution, he said.
The two said they were avenging the Prophet Mohammed and shouted "Allahu akbar," which translates to "God is great," Paris Prosecutor Francois Molins said.
Cell phone cameras caught the two gunmen as they ran back out of the building, still firing. One of them ran up to a wounded police officer lying on a sidewalk and shot him point-blank.
It was the deadliest attack in Europe since July 2011, when Anders Behring Brevik killed 77 people in attacks on government buildings in Oslo, Norway, and at a youth camp on the island of Utoya.
But it won't stop Charlie Hebdo. Pelloux told CNN affiliate BFMTV that thousands of copies of the magazine will be published next Wednesday. Proceeds from the issue will go to victims' families, France's Le Monde newspaper reported.
'It was their only mistake'
Police hunting for the Kouachi brothers have searched their supposed residences in a number of towns, Interior Minister Bernard Cazeneuve said.
The two masked men apparently left behind a getaway car, which police impounded. Investigators found Said Kouachi's identification card inside the car as they combed it for evidence, Cazeneuve said.
"It was their only mistake," said Dominique Rizet, BFMTV's police and justice consultant, reporting that the discovery helped the investigation.
The Kouachi brothers returned from Syria in the summer, USA Today reported, without saying where it got the information.
Officials were running their names through databases to look for connections with ISIS and al Qaeda. The suspects were known to security services, Prime Minister Valls said.
A third suspect, 18-year-old Hamyd Mourad, turned himself in to police, a source close to the case told the AFP news agency. In French media and on social media, classmates of Mourad, who is in his final year of high school, said he was with them at school at the time of the attack.
Cazeneuve said that nine people overall have been detained in connection with the Charlie Hebdo attack.
But that doesn't necessarily mean authorities are any closer to Cherif and Said Kouachi.
'Parisians will not be afraid'
The victims' names were splashed Thursday across newspapers as heroes for freedom of expression. "Liberty assassinated." "We are all Charlie Hebdo," the headlines blared.
They included two police officers, Stephane Charbonnier -- a cartoonist and the magazine's editor, known as "Charb" -- and three other well-known cartoonists known by the pen names Cabu, Wolinski and Tignous. Autopsies on the victims were underway Thursday, Cazeneuve said.
Flags flew at half-staff on public buildings and events were canceled Thursday, a national day of mourning. Crowds gathered in the rain in Paris in the victims' honor, many holding up media credentials and broke into applause as the silence ended. The bells of Notre Dame Cathedral tolled across the city.
"I can't remember such a day since 9/11," said Klugman, Paris' deputy mayor. "The country really is in a kind of shutdown in respect and memory of the 12 people killed."
The day earlier, thousands poured into streets in hordes in a show of solidarity, holding up pens and chanting, "We are Charlie!" Similar demonstrations took place in cities in addition to Paris, including Rome, Berlin and Barcelona.
According to Klugman, "Parisians will not be afraid."