In 1993 six people were killed at the World Trade Centre in New York.

Watch Mark Urban's report

Diplomatic Correspondent, Mark Urban, says the attack established the hallmark of a new form of terror - large homemade bombs set off with no warning, by militants acting with very little state support.

A foiled attack in Bangkok, Thailand, in 1994 had the same hallmarks - a homemade bomb so large it needed to be carried by a truck. It was intended for the Israeli embassy.

Scene of the 1996 Al Khobar bomb

In 1995 in Saudi Arabia, five people were killed in an attack on US forces in Riyadh.

And the following year, 19 US air force staff were killed by large truck bomb in Al Khobar.

Ex-Secretary of State, John Bolton

John Bolton, former US Assistant Secretary of State, says he expects to see more bombings of the type seen in Kenya and Tanzania.

So far there is no hard evidence pointing to Islamic extremists. But the bombing has been welcomed by some Muslim groups.

Abu Hamzeh

UK-based Egyptian cleric Abu Hamzeh says: "The Americans jumped the queue of enmity against Islam and they just have to have these couple of slaps to tell them to go back."

Militant Islamists bear many grudges against America.

They despise US for its support of Israel and say the presence of US forces in Saudi Arabia has desecrated Islam's holiest shrines.

John Bolton says: "I think the US is viewed as principal target of Islamic terrorists who are trying - at a minimum - to get the US in effect to withdraw from the Middle East."

New breed of terrorist?

Maher Othman: Nature of terrorism changed drastically

Maher Othman, of the Arabic newspaper, Al Hayat, says many terrorist organisations are based outside their respective countries, and are so widespread that it is difficult to trace them.

Lawrence Korb: This problem cannot be solved by a single state

Former US assistant Secretary of Defence, Lawrence Korb says the informal structure of such groups can lead to intelligence problems.

"These groups cut across state lines. They are not beholden to any state and they also are getting a lot of private money," he says.