Al Qaeda gunmen Said Kouachi, 34, and his brother, Cherif, 33, have reportedly told police negotiators: 'We are ready to die as martyrs'.
The press understands police scrambled phone signals in the area after the gunmen made contact with a fellow jihadist while inside the building.
Officers are hunting the associate who is feared to be behind the murder of a police officer yesterday and a new hostage crisis ongoing in Paris today.
The gunman, named as Amedy Coulibay, 32, this afternoon took at least five others hostages at a kosher grocery in the east of the capital.
He is believed to be working with his girlfriend Hayat Boumeddiene, 27, who is also said to be 'armed and dangerous'.
Coulibay is thought to be demanding that the Kouachi brothers be allowed to go free in return for the release of his captives, according to reports.
Revelations that a call was made by the Kouachis suggests they may have instructed their accomplice to carry out today's atrocity. Local media reports that the brothers met the third hostage-taker while in prison.
He is believed to be a fellow member of the Buttes Chaumont – a gang from the 19th arrondissement of Paris that sent jihadists to fight in Iraq.
The Kouachis were cornered in the village of Dammartin-en-Goele, around 25 miles from the capital, after leading police on a dramatic car chase.
After exchanging gunfire with officers, they fled on foot into an industrial estate where they are holding a hostage, believed to be a 26-year-old male.
Snipers have their weapons trained on the building and helicopters are hovering overhead as negotiations were underway with the Islamic fanatics.