As one gunman ran out of the office of Charle Hebdo magazine in Paris, he raised his right index finger in a gesture that has been adopted by radical Islamists, including al-Qaeda.
The single finger alludes to the "tawhid", namely the Muslim belief in the oneness and unity of God. It can be taken to invoke the first half of the Muslim statement of faith: "There is no God but Allah and Mohammed is his messanger".
The gunman is shown striding towards the camera, raising his right finger and carrying an AK47 assault rifle in his right hand.
The attackers who killed 12 people in and around the office of Charlie Hebdo magazine in Paris acted with a skill and calmness that bears all the hallmarks of advanced military training.
They were also fully equipped for their murderous task. Photographs taken at the scene show two men clad entirely in black, their faces concealed by balaclavas. Each one is armed with an AK47 automatic rifle.
They wear army-style boots and have a military appearance and manner. One of the men wears a sand-coloured ammunition vest apparently stuffed with spare magazines. Some reports suggest that an attacker was also carrying a rocket-propelled grenade launcher.
The men attacked the magazine's headquarters with clinical precision, killing their victims and then shooting two police officers in the street outside.
Amateur footage shows them using classic infantry tactics. They move along the street outside the office working as a pair: one advances while the other gives cover.
Instead of spraying automatic gunfire, they fire two aimed shots at each target - a pattern known as "double-tap" firing - thereby conserving their ammunition.
According to one witness, the terrorists shouted that they were from "al-Qaeda in Yemen". Cédric Le Béchec, a 33-year-old estate agent, told a French news website that the attackers approached another man in the street saying: "Tell the media that this is al-Qaeda in Yemen". Yemen is the base of one affiliate of the terrorist network usually known as "al-Qaeda in the Arabian Peninsula".
Other shocking footage shows the moment when they realise that one policeman is lying wounded on the pavement. They calmly approach the injured victim and, while one terrorist gives cover, the other kills the officer with a shot to the head.
Then the attackers are shown driving away in a black Citroen DS. They do not race off in a way that would draw attention; instead they depart in a controlled manner. Later, they tried to cover their tracks by abandoning this vehicle and hijacking a second car.
The fact that they were clearly following a careful escape plan - which appears to have worked so far - is further evidence of their ability.