The makeshift bombs lack the destructive potential of the conventional explosives that are touched off in Iraq on a daily basis. They are also less reliable, as demonstrated by the car bombs that failed to go off in London last week after the culprits tried to ignite them with detonators wired to mobile phones.

But other attempts have generated plenty of mayhem and damage, including the kitchen-built backpack bombs that killed 52 people in the London public transit system July 7, 2005.

“It makes no difference to your average person if somebody puts a car bomb out there that is crude or one that is sophisticated,” said Chris Driver-Williams, a retired British major and military intelligence officer who studies explosive devices used by terrorist groups. “If it detonates, all of a sudden you’ve got a very serious device and one that has achieved exactly what the terrorists wanted.”

The advantages of homemade explosives are that they are easy and cheap to manufacture, as well as difficult for law enforcement agencies to detect. According to one expert, the peroxide-based liquid explosives that an al-Qaeda cell allegedly intended to use to blow up nine transatlantic airliners last summer would have cost as little as $15 a bomb. It is also technically simple to make such explosives. Instructions are widely available on the Internet.

Investigations have found evidence that most al-Qaeda cells involved in bombing plots in Europe received training in camps in Pakistan or Afghanistan.