In the battlefields of Syria and Iraq, the Islamic State group became infamous for its spectacular variations on explosive vehicles. For attacks in the West, it has suggested a simpler method, encouraging followers to use regular vehicles to kill people on foot.

Experts say attacks in which cars or trucks are driven into popular pedestrian areas present a unique challenge for law enforcement officials as they are nearly impossible to predict and easy to pull off. They require no advanced training, no specialized materials. Almost anyone can own or rent a vehicle.

Some feel that these low-tech, lone wolf operations can have the same psychological impact as larger, more sensational attacks.

Four people were killed and dozens wounded Wednesday in London with this tactic — the worst attack on British soil since the transport network bombings on July 7, 2005.

Charlie Winter, a senior research fellow at the London-based International Centre for the Study of Radicalisation and Political Violence, says what makes such attacks so frightening is the relatively low barriers to entry. The method was embraced by al-Qaida before being revitalized by IS.

"It makes for a very effective unsophisticated high impact, very frightening form of an operation," he said. "You don't need to know someone who can make you a bomb or buy you a gun in order to carry out an attack. It's a very difficult thing to fight against. There is no quick fix."

Read more:  AP