The two main suspects in the Islamist attack on Charlie Hebdo magazine in Paris are said to have robbed a service station in the north of France.
They stole food and petrol, firing shots as they struck at the roadside stop near Villers-Cotterets in the Aisne region, French media report.
France has observed a minute's silence for the 12 people killed at the office of the satirical magazine.
Earlier in the day, a gunman shot dead a policewoman in southern Paris.
A second person was seriously injured in the attack in the suburb of Montrouge, after which the gunman fled.
It is unclear if the attack is related to the pursuit of prime suspects Cherif and Said Kouachi.
According to the manager of the service station that was robbed on the RN2 road in Aisne at about 10:30 (09:30 GMT), the attackers fit the description of the two men, and were heavily armed with Kalashnikovs and rocket-propelled grenade launchers.
They are said to have driven off in the direction of Paris in a Renault Clio car, apparently the same vehicle hijacked in Paris soon after the Charlie Hebdo attack.
Those killed (from left) include economist Bernard Maris, prominent cartoonists Wolinski and Cabu, Charlie Hebdo editor Stephane Charbonnier and cartoonist Bernard Verlhac
Source: Le Monde newspaper and other French media
During the national silence, the bells of Notre Dame cathedral in the capital tolled in mourning.
French Interior Minister Bernard Cazeneuve warned against jumping to conclusions after the pre-dawn shooting in Montrouge.
The gunman was armed with a machine-gun and a pistol and wore a bullet-proof jacket, police sources told AFP news agency.
A local resident, Ahmed Sassi, described a "scene of panic". He said he had seen a police officer standing and then a man dressed in dark clothes who ran up and shot the officer "at point black range".
"I saw the officer fall and a colleague call for help," Mr Sassi said.
Overnight, seven people believed to be connected to the Kouachi brothers were detained in the towns of Reims and Charleville-Mezieres, as well as in the Paris area.
Cherif Kouachi was sentenced in 2008 to three years in prison for belonging to a Paris-based group sending jihadist fighters to Iraq.
A man was also seriously injured in the shooting
Police officers assisted a woman at the scene of the shooting in Montrouge
Armed police deployed in Reims overnight
The French flag is flying at half-mast over the Elysee Palace in Paris
French President Francois Hollande presided over an emergency cabinet meeting in Paris on Thursday
AFP staff held up "I am Charlie" placards at the French news agency's Hong Kong office
A vigil for Charlie Hebdo victims was being held in Melbourne, Australia, on Thursday
'We killed Charlie Hebdo'
Paris has been placed on the highest terror alert and extra troops have been deployed to guard media offices, places of worship, transport and other sensitive areas.
On Wednesday, eight journalists - including the magazine's editor - died along with a caretaker and a visitor when masked men armed with assault rifles stormed the Charlie Hebdo offices during an editorial meeting. Eleven people were also wounded, some seriously.
Two policemen were also killed.
Witnesses say the gunmen shouted "we have avenged the Prophet Muhammad" and "we killed Charlie Hebdo", as well as "God is Great" in Arabic.
The attackers fled to northern Paris before abandoning their car and hijacking a Renault Clio, police say.
The magazine's office was firebombed in 2011. It had angered some Muslims by printing cartoons of the Prophet Muhammad as part of its irreverent take on news and current affairs.
Vigils were held through the night in Paris and cities worldwide in tribute to the dead. Many demonstrators held up placards reading "Je suis Charlie" (I am Charlie) in solidarity with the victims.
French President Francois Hollande said the country's tradition of free speech had been attacked and called on all French people to stand together.
Piles of pens - symbolising freedom of expression - and candles were laid across the Place de la Republique square in Paris where thousands of people had gathered.
Cartoon tributes are circulating on social media, sending out the message of press freedom. One Dutch cartoon plays on 9/11 Twin Towers imagery, showing a plane flying towards two upright pencils.
Thursday's national day of mourning is only the fifth held in France in the past 50 years.
National Days of Mourning in France
Charlie Hebdo attack sequence