A manhunt to trace two brothers wanted over the Paris magazine massacre was focusing on a stretch of countryside outside the capital on Thursday. There were reports of a heavy police presence around the thickly wooded area near a petrol garage where Said and Cherif Kouachi were apparently seen earlier today.
The pair are the main suspects in Wednesday's deadly attack on the offices of satirical newspaper Charlie Hebdo. The tension gripping France deepened further on Thursday after a policewoman was shot dead in a southern Parisian suburb this morning in an attack which officials are believed to be treating as a terrorist act.
Earlier on Wednesday, the first image of the bloodied offices of the newspaper was published in the French daily Le Monde, showing a hallway smeared with blood. Blood-stained papers also lay on the ground, hinting at the chaos and panic that unfolded as masked gunmen burst into the offices armed with Kalashnikovs and opened fire.
Security has been stepped up at ports and borders in the UK following the attack. Thousands of police and security officers have been deployed to hunt for the brothers and the search 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 were scouring the dense woodland in the 13,000 hectare Foret de Retz around 50 miles (80km) outside Paris, while searches were carried out in the picturesque towns of Villers-Cotterets, Longpont and Corcy.
Benoit Verdun, a hotel worker in Longpont, told Sky News: "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.
Interior Minister Bernard Cazeneuve said a total of nine people are now in custody and more than 90 witnesses have been interviewed. He said that "all means" available are being deployed to trace the fugitives. He also confirmed that agencies have so far established no link between the Charlie Hebdo attack and this morning'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 yesterday raised 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 Charlie Hebdo'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.
Cherif Kouachi was convicted of terrorism charges in 2008 for helping funnel fighters to Iraq's insurgency and sentenced to 18 months in prison, according to reports. 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 sense of unease in Paris was compounded by the second fatal attack on police in consecutive days. Thursday's shooting took place early this morning 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 stand ready to support their French counterparts' search for those suspected of the Charlie Hebdo attack.
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.
"We continue to review our ability to prevent and respond to terrorist incidents and we have deployed additional measures to help reassure and protect the public. The measures we take to protect the public are constantly under review. There is a lot of proactive work going on by the National Counter Terrorism Policing Network and other agencies to mitigate the threat from terrorism."
Wednesday's killings drew condemnation around the world and French President Francois Hollande declared a national day of mourning. At midday local time a minute's silence was observed across the country, with quiet descending on public squares, schools and on Metro trains. Tonight the Eiffel Tower went dark in tribute to the victims of the attack.
Police forces across Britain paid tribute to the two French officers murdered in the magazine attack by pausing "in solidarity and sympathy" this morning.