CRÉPY-EN-VALOIS, France — Authorities swarmed French villages, scoured surveillance video, questioned witnesses and tried to soothe a nervous country on Thursday as they hunted for two brothers suspected in the massacre at Charlie Hebdo magazine.
Armored cars and scores of police officers and anti-terror forces descended on quiet towns and farmhouses in hopes of capturing Cherif and Said Kouachi, suspected of killing 12 people the day before and speeding away into the streets of Paris.
In a mostly rural patch of France about an hour outside the capital, officers converged and helicopters hovered over several villages after two masked robbers with machine guns held up a gas station on Thursday morning. It was being treated as the last known sighting of the suspects, an investigator said.
Police operations were carried out in Crépy-en-Valois, Corcy and Longpont. They included the military's GIGN special forces unit, dressed in fatigues and ski masks and carrying camouflaged rifles.
"They went room by room looking for anybody hiding," Lassad Toumi told NBC News after he came home from feeding his horses and was stopped by SWAT teams who said they wanted to search his home for his own safety.
The manhunt included thousands of police and investigators, the prime minister told RTL radio.
Police March into Longpont in Search for Charlie Hebdo GunmenJan. 8, 201500:51
One day after the massacre, France faced the difficult task of mourning its worst terrorist attack in decades and dealing with the unsettling fact that the two men believed to have carried it out could be anywhere.
Adding to the fears, someone shot a police officer to death and wounded another on Thursday morning in a Paris suburb and then fled. There was no apparent connection to the massacre at Charlie Hebdo, but authorities referred it to an anti-terror unit for investigation and did not rule out a link.
Interior Minister Bernard Cazeneuve told reporters Thursday evening that the government had taken additional steps to secure public safety, including mobilizing 88,000 security officers, with 4,000 military personnel to be added.
He said that the country was not frightened and would be united in the face of tragedy.
At a juice bar in the Saint-Lazare train station, Laura Stern, a Parisian student, told Reuters that she was "a little bit scared" but would not stop working or going out in public.
"People here keep coming up to me to say: be careful," she said. "But that's not a reason to stay at home."
Cazeneuve said that investigators were carefully analyzing telephone and Internet traffic and video from surveillance cameras. He said that 90 witnesses had called in to a special phone line.
He also said that the Kouachi brothers' parents had been questioned on Wednesday and that nine other people had been detained in the investigation.
The younger brother was one of seven men convicted in 2008 of helping to funnel fighters to Iraq as part of an insurgent cell. He was sentenced to a year and a half in prison.
Eyewitness Video Shows Paris GunmenJan. 7, 201500:28
The record raised the question of how Cherif Kouachi, at least, escaped the attention of authorities. "These people were being watched over but there were no elements at the time to warrant starting an inquiry," the interior minister said.
In Washington, President Barack Obama was getting regular briefings, and the Justice Department said that Attorney General Eric Holder would travel to Paris on Sunday for an anti-terror meeting convened by France because of the attack.
In France, Thursday was observed as a national day of mourning on orders from the president. A moment of silence was observed at noon. Hundreds stood in the rain at Notre-Dame cathedral, and the Paris subway stopped rolling.
An outpouring of grief and rage continued around the world. People gathered for vigils after the attack in major cities, many of them holding placards that said "Je Suis Charlie," or "I am Charlie."
The magazine has satirized Judaism and Christianity as well but has been targeted in the past for sendups of Islam, including caricatures of the Muhammad, the central figure of the Muslim faith.
Among the victims of the attack were the top editor, Stephane Charbonnier, and a prominent French economist who contributed. In all, eight employees or contributors were killed, as were a guest, a maintenance worker and two police officers.
The assault triggered fears of reprisals against French Muslims. At least three mosques reported violence on Wednesday or Thursday, including one where shots were fired from outside and another were a grenade went off. No one was reported hurt.
Police investigate Armed Robbery at Gas Station in FranceJan. 8, 201500:33
NBC News' Paul Nassar and Nancy Ing contributed to this report. Alexander Smith and Paul Nassar reported from London.