Bombs and guns aside, a smartphone can be a powerful weapon in the hands of a terrorist - but it can also provide intelligence services with the tools to track them down. Three years ago to the day, the Paris attacks of November 13, 2015 remain one of the best known examples of a large-scale assault that could not have been planned without phones.
The Islamic State group gunmen and bombers who struck the Bataclan concert hall and other nightlife spots used them extensively to coordinate the carnage, said a former French anti-terrorist official, speaking on condition of anonymity.
Just before entering the Bataclan, where they massacred 90 people, the attackers had sent a text message to accomplices in Belgium: "We're going ahead. It's started."
But if smartphones have been a "game-changer" for jihadists, their use by the world's extremists goes much further back than the Paris attacks.
"As of 2003, in Iraq, home-made bombs started being set off by the sending of an SMS as American convoys drove past. This caught on and was then repeatedly used by Al-Qaeda," the ex-official told AFP.
These days, encrypted apps such as Telegram, Wire and WhatsApp can help jihadists communicate while evading police tracking - or at least complicate efforts to decode their messages.
For several years IS has published online tutorials in several languages explaining to jihadists how to choose the best software to evade detection in war zones.
For new recruits in developing countries, where smartphones are more common than computers, there are different strategies still.
"Phones are no longer phones - they're computers," said Laurent Heslault, director of security strategies at Symantec, a security group.
"They are far more powerful than what we had on our desks 10 years ago," he added.
"They have more computing power, more memory and connection capabilities. They are very powerful tools when it comes to communicating."
That has also made it much easier for jihadist groups to recruit new members.
Smartphones "enable people to reach out for propaganda" with the swipe of a screen, said the retired official.
"Thirty years ago, guys used to exchange video cassettes, then it was CDs. Now it's online and can be looked up at any time."
For propaganda-makers, videos of attacks can be filmed and uploaded in the blink of an eye.
"You can film attacks, claim responsibility, use (a phone) to take photos and film reconnaissance operations," the ex-official said.
But the smartphone can be an extremist's downfall as well as their best asset.
Intelligence agencies have grown better at using phones to identify suspects, spy on them - and, in case of capture, lift data for use as evidence in court.
That in turn has raised difficult questions for tech giants who promise their users privacy.
Most famously, Apple faced a court showdown with the FBI after agents sought access to the data of the attackers who killed 14 people in San Bernardino, California, in December 2015. Investigators dropped the case after finding a way into the phone without help from Apple, which argued that helping authorities access a phone would set a dangerous precedent.
Further afield, governments have used phone data extensively to pinpoint extremist suspects.
The French military intervention in Mali, launched in 2013 after jihadists took over the northern half the country, started with air strikes whose targets were chosen based on phone data, the former French official said.
"Today all air strikes focus on telephones," he added.
"Even if you keep changing the SIM card the phone has its own identity and once detected can continue being tracked."
And when it comes to police investigations, smartphones sometimes provide more information than their owners.
They might allow investigators to work their way back along an information trail, snare other members of a suspect's network, and track sleeper cells, he added.
"Smartphones make you a target," the expert said.
"Because of this jihadist leaders have learned to keep away from them. For the past few years, there's been a marked return to using human envoys," he added.