BY NANCY ING, ED KIERNAN, CASSANDRA VINOGRAD AND ALASTAIR JAMIESON
This is a developing story. Please check back for updates.
DAMMARTIN-EN-GOELE, France — A French town was circled by anti-terror forces Friday after two brothers being hunted over the Charlie Hebdo massacre apparently seized a hostage.
WATCH: Operation underway to detain French suspects
Paris police spokesman Xavier Castaing told NBC News that an operation was underway in Dammartin-en-Goele as authorities tracked "armed and dangerous" fugitives Cherif and Said Kouachi.
He added that shots had been fired and at least one hostage taken at a building. The town of about 8,000 is located about 20 miles northeast of the French capital and just five miles from Charles de Gaulle Airport.
Police surrounded a family-run printing company in a mostly industrial section in Dammartin-en-Goele and helicopters hovered overhead.
"There are indications the terrorists have been spotted," French Interior Minister Bernard Caseneuve told a news conference. He confirmed that an operation had been launched to detain the gunmen who attacked the satirical magazine's Paris offices on Wednesday, shouting "Allahu Akbar'' — or "God is greatest."
Their 12 victims included a Muslim police officer who was murdered at point-blank range in the street.
Interior Ministry spokesman Pierre-Henry Brandet later told iTELE television that officials were "almost certain" that the Charlie Hebdo suspects had been cornered in the building.
WATCH: Reports of shots fired, hostages taken in France
The nearby Charles de Gaulle School had been locked down, student Marion Genay told NBC News via Twitter. "Dammartin is surrounded by the [armed police]," she said. "There is a very tense atmosphere, being locked in the high school."
Local residents were to told to stay away from windows, turn the lights off and stay indoors.
"It's basically a war zone in Dammartin-en-Goele," one witness told BFM TV.
The Associated Press quoted a French security official as saying the two suspects had earlier stolen a car amid gunfire in Montagny Sainte Felicite, which is about five miles from Dammartin-en-Goele. NBC News could not confirm that report.
Paris' Charles de Gaulle Airport told NBC News that some aircraft coming into land had been redirected to its southern two runways, which are further away from Dammartin-en-Goele. "The town is a little bit too close to the airport," a spokesman said.
Nearly 100,000 security personnel were on alert across France on Friday.
Police are aware that at least one of the Kouachi brothers has had military training.
Said Kouachi, 34, traveled to Yemen in 2011 to be trained by al-Qaeda in the Arabian Peninsula, two senior U.S. counterterrorism officials told NBC News. AQAP is considered the most violent branch of al Qaeda.
Cherif Kouachi, 32, was sentenced to prison in 2008 after a Paris court found him and six other men guilty of helping funnel fighters to Iraq.
A Homeland Security official told NBC News that the brothers had been on the U.S. no-fly list "for years."
On Thursday, elite anti-terrorist forces converged on several villages after two masked robbers with machine guns matching the Kouachi brothers' description held up a gas station in Villers-Cotteret, France.
Charlie Hebdo's offices had been firebombed in the past after publishing cartoons of the Prophet Muhammad. The publication's chief editor — who was known as Charb — was an outspoken supporter of free speech who had reportedly been put on an al Qaeda hit list. He was among those slain.
NBC News' Alexander Smith, Nikolai Miller and Jason Cumming, The Associated Press and Reuters contributed to this report.