The massive manhunt for two brothers believed to have carried out the Paris massacre has entered its third day.

Police have focused their search on a stretch of countryside outside the capital near a petrol garage which Said and Cherif Kouachi reportedly robbed yesterday.

For the second night Parisians flocked to the Place de la Republique for a vigil to remember the 12 people killed in the attack on satirical magazine Charlie Hebdo.

And the Eiffel Tower was plunged into darkness in tribute to those slaughtered.

The brothers were already known to US authorities and had been put on the American no-fly list, a senior US counter-terrorism official said.

Another US official said the older brother, Said had travelled to Yemen. It was unclear whether he was there to join up with extremist groups such as al Qaida.

A third man, Hamyd Mourad, 18, surrendered to police, after hearing his name on the news in connection with the attack, a judicial official said.

The tension gripping France deepened further yesterday after a policewoman was shot dead in a southern Parisian suburb in an attack which officials are believed to be treating as a terrorist act.

Thousands of police and security officers have been deployed in the hunt, which is concentrating on a rural area to the north of the city after reports that they were spotted driving a Renault Clio at a roadside petrol station in the Aisne region.

Teams of heavily armed officers have been scouring the dense woodland in the 13,000 hectare Foret de Retz around 50 miles outside Paris.

And searches were carried out in the picturesque towns of Villers-Cotterets, Longpont and Corcy.

Benoit Verdun, a hotel worker in Longpont, told Sky News yesterday: "There are lots of policemen. I can see a huge police car. They are asking people 'Have you seen anybody?' They have big guns with them.

"The forest is bigger than Paris - it is very big and very wide."

He said police asked him to close the hotel and stay inside.

Last night interior minister Bernard Cazeneuve said a total of nine people were now in custody and more than 90 witnesses had been interviewed.

He said "all means" available were being deployed to trace the fugitives. He also confirmed that agencies have so far established no link between the Charlie Hebdo attack and yesterday's shooting.

The nationwide search for the brothers appeared to be narrowing after they were identified by the manager of a filling station.

They stole food and petrol from the service station and were hooded and armed with Kalashnikovs, it was claimed.

The Vigipirate plan - the French national security alert system - was raised on Wednesday to its highest level, "alerte attentat", across the entire Ile-de-France region around Paris.

That threat level has now been extended to cover Picardy, the northern region where the suspects are said to have been spotted.

In total more than 88,000 security personnel have been deployed across the region to reflect the heightened alert.

The manhunt was launched after masked gunmen stormed into the magazine's offices and opened fire, killing eight journalists, two police officers, a maintenance worker and a visitor in France's worst terror atrocity since 1961.

They shouted "We have avenged the prophet" after the shooting. The weekly publication had been threatened before for its caricatures of the Prophet Muhammed.

The sense of unease in Paris was compounded by the second fatal attack on police in consecutive days.

Yesterday's shooting took place when an officer stopped to investigate a traffic accident. A street sweeper was critically injured in the incident.

Witness Ahmed Sassi said: "There was an officer in front of a white car and a man running away who shot."

The gunman, who was dressed in dark clothes, fled after the shooting in Montrouge, just to the south of the city.

Two explosions were reported near mosques south of Paris. No one was hurt in those incidents.

Meanwhile, security has been stepped up at UK ports and border controls. The measures have been put in place "on a precautionary basis" but there has been no change to the threat level in the UK, Downing Street said.

Andrew Parker, the head of MI5, said the Charlie Hebdo attack was a "terrible reminder of the intentions of those who wish us harm".

The director-general of the Security Service said MI5 would be offering its French counterparts its "full support".

Assistant Commissioner Mark Rowley, Britain's most senior counter-terrorism officer, said UK police stood ready to support the French search.

He said: "A massive manhunt is under way to catch the suspects and UK police will continue to do all they can to assist our colleagues in France to help catch the people who carried out that attack.

"At this stage, there is no UK connection and the threat levels remain unchanged, at severe for the UK."

Wednesday's killings drew condemnation around the world and French president Francois Hollande declared a national day of mourning for yesterday.

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