Gunmen believed to be behind the attack on French magazine Charlie Hebdo entrenched themselves Friday with a hostage inside a print facility northeast of Paris, police officers and witnesses said.
Police swarmed the industrial area in a town not far from the Charles de Gaulle airport, as helicopters loomed low overhead.
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"An operation is currently under way near Dammartin-en-Goële that is mobilizing all the forces in the area," French Interior Minister Bernard Cazeneuve said in a short televised address.
Tension spiked Friday, when two gunmen believed to be the suspected shooters seized a Peugeot car in a forest area north of Dammartin-en-Goële. Thousands of policemen had been combing that area all night after witnesses had alerted authorities about the suspects' possible presence in Villers-Cotterêts.
The gunmen drove south toward Paris before clashing with police forces when crossing through Dammartin-en-Goële, according to a police official.
Tens of thousands of soldiers and police mobilized across France to find the two suspects after President François Hollande raised the terror alert in an area of the search for the men believed to have methodically shot 12 people at the satirical magazine.
"The effectiveness of the measures put in place yesterday allow us to act, and act quickly," Mr. Cazeneuve said.
U.S. and French intelligence believe that one of the two gunmen--both of whom have been on watch lists of possible terrorists for years-- had received weapons training from an al Qaeda offshoot in Yemen in 2011, U.S. officials said.
The two suspects were identified as 34-year-old Said Kouachi and his brother, Chérif Kouachi, 32, both French citizens.
Dammartin-en-Goële, a town of about 8,000 people, is located within commuting distance of Paris and is barely a 15-minute drive northeast of Charles de Gaulle, France's biggest air-transport hub.
It has three industrial zones, including Les Prés-Boucher where the suspects are believed to be holed up. The business in Les Prés-Boucher are mainly logistics operations for groups including national postal service La Poste and retailer Aldi Marché. The site is at the north of the town, close to roads connecting it to Paris.
The town mayor's website carried a statement ordering residents to stay in their homes. Children will be kept in schools and the school buildings will be secured, the mayor's office said.
Television showed pictures of police sealing the off the town and helicopters in gray, rainy skies circling.
"For the past two hours, helicopters have been flying low over the area, which is now completely deserted," said Jaona Ravelohari, who lives a few blocks away. "The police has asked us to close our shutters, and stay in," he added.
"I was drinking a coffee with a colleague and then we saw the gendarmes arrive in cars," Christelle Alleaume said in a telephone interview on iTele. Ms. Alleamue said she believes the company where the suspects are holed up is a print shop. "They ran out of the cars and then we heard shots fired, so went back inside very quickly."
"There are lots of helicopters that are flying very low; one helicopter was so close it seemed we could touch it," she said.
Operations at Charles de Gaulle were slightly disrupted Friday morning by the low-altitude police helicopters. One of the airports four runways, used for arrivals, was closed and all arrivals were diverted to the other landing strip, but because of light traffic midmorning the shift didn't cause delays, said a spokesman for Aeroports de Paris SA, the airport's owner.
Flights of Air France-KLM SA, the largest airline at the giant airport, were operating normally, said an Air France spokeswoman. She said some flights may have been told to go around for a second landing approach due to the search.
Mr. Cazeneuve said Thursday that both the suspects were known to French security and were under surveillance, but no incriminating evidence had been gathered on them.
The minister said the elder brother had been formally identified from a photograph as one of the attackers. His national identity card was found in an abandoned Citroën that had been used as a getaway car.
While Said Kouachi had no police record, his brother had been sentenced to 18 months in prison in 2008 for being part of a terror group, prosecutors said.
Both have been in a U.S. database of suspected terrorists--and on the U.S. no-fly list--for years as well. For the younger brother, that was due to his conviction in France, while the other's name had surfaced in other terrorism-related probes, according to U.S. officials.
A third suspect identified by police as Hamid Mourad, 18, turned himself in late Wednesday. His relationship to the others was unclear.
The masked gunmen's attack decimated Charlie Hebdo's newsroom, which has for years stirred outrage--and death threats--with its caricatures lampooning Islam, just one of its many targets of satire.
Dan Michaels and Blandine Hénault contributed to this article.
Write to William Horobin at William.Horobin@wsj.com and Jason Chow at firstname.lastname@example.org