Reports emerged today that two suspects being hunted over the atrocity at the Charlie Hebdo magazine's offices have been spotted at a petrol garage in a region to the north of the capital.
Police are also searching for the perpetrator of a separate shooting in which a policewoman was killed just outside Paris this morning. It was unclear whether there was any link between the two incidents.
Meanwhile, security has been stepped up at ports and border controls in the UK following yesterday's assault.
Downing Street said the measures had been put in place "on a precautionary basis" but there had been no change to the threat level in the UK.
Police and security forces in France are scouring the country for brothers Said and Cherif Kouachi.
The operation was launched after masked gunmen stormed the Paris offices of Charlie Hebdo, killing 12 people in France's worst terrorist atrocity since 1961.
Local media said two suspects in the attack were seen armed and getting into a Renault Clio in Aisne, a region to the north-east of the capital.
The sighting was said to have been reported to police by a worker at a motorway filling station.
Yesterday's killings drew condemnation around the world and France's president Francois Hollande declared a national day of mourning today.
At midday local time a minute's silence was observed across the country, with quiet descending in public squares, schools and on Metro trains.
Police forces across Britain paid tribute to two French officers murdered in the magazine attack by pausing "in solidarity and sympathy" this morning.
Today's attack took place early this morning when an officer stopped to investigate a traffic accident. The second casualty was a street sweeper.
Witness Ahmed Sassi said: "There was an officer in front of a white car and a man running away who shot," adding that the gunman was dressed in dark clothes.
The perpetrator fled and is still at large, France's senior security official Bernard Cazenueve said. He travelled to the scene after leaving an emergency government meeting.
