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Islamic State group claimed responsibility for the attack that rocked London Friday — the fifth this year — and UK officials raised the terror threat to critical, meaning they expect another attack, reports the Los Angeles Times .

The attack sent panicked commuters running for cover during the morning rush hour and sparked a massive manhunt for the perpetrator, who remained at large.

Authorities said the incident was caused by an improvised explosive device that had been left inside a bucket in a supermarket cooler bag on a busy subway car heading toward central London. Photos on social media showed a white bucket on fire inside the train.

The bomb reportedly had a timer attached, but it was not immediately clear whether it detonated as planned or earlier than intended.

Twenty-nine people were injured, most suffering flash burns, but no one was killed.

The mayor of London, Sadiq Khan, called for more police resources in London, saying he was unapologetic for using this incident to again spotlight the issue of budget cuts.