LONDON — An Iraqi teenager who arrived in Britain as an asylum seeker was found guilty on Friday of a bombing that sent a fireball through a London subway train last year, a case that raised questions about the country’s anti-radicalization efforts.

A court in London convicted the teenager, Ahmed Hassan, 18, of attempted murder for leaving an explosive device, concealed in a bucket and a supermarket plastic bag, on a train at the Parsons Green Station in west London on Sept. 15.

The explosion at the height of the morning rush and the resulting panic left 30 people injured, some with serious burns, but none were killed.

Image Ahmed Hassan Credit... London Metropolitan Police

The bombing was the fifth terrorist attack in Britain that year and the first time the capital’s public transportation network had been targeted since the deadly assaults of July 2005. In response, British officials briefly raised the country’s terrorism threat level to the highest possible level.