Story highlights Peter Bergen: Key question for Manchester bombing: what kind of bomb? If hydrogen peroxide (a good possibility) how was it built, concealed?

He says building such a bomb would imply training, as chemicals are unstable, need careful handling

Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of "United States of Jihad: Investigating America's Homegrown Terrorists."

(CNN) As investigators look for key clues in the Manchester bombing, one of the most important questions to answer will be: Was this a hydrogen peroxide-based bomb? And if so, where was it constructed? And, how did the Manchester perpetrator learn how to build his bomb?

To recap: Manchester is reeling from the worst terrorist attack in the United Kingdom since July 7, 2005 when al Qaeda-trained suicide bombers killed 52 commuters on London's transportation system.

The attack in Manchester Monday at the Ariana Grande concert killed at least 22 and was carried out by a suicide attacker, officials said.

Suicide attacks are quite rare in the UK. Indeed, the Manchester attack is the first suicide attack since the London transportation attacks 12 years ago.

Like school shooters, terrorists study previous attacks. The attack at the Ariana Grande concert may well have taken some of its cues from the ISIS-directed attack on the Bataclan theater in Paris, where an American rock band, the Eagles of Death Metal, was playing on November 13, 2015. The ISIS-trained attackers killed 89 at the theater.

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