The teenager who killed three young girls and wounded 10 other people in a knife attack on a dance class in Southport, England, last summer will be sentenced on Thursday.

Judge Julian Goose, who is presiding over the case, told the attacker, Axel Rudakubana, 18, that a life sentence equivalent would be inevitable after he pleaded guilty on Monday.

Since that court hearing, a portrait of a deeply troubled young man obsessed with violence has emerged, as has the fact that he was on the radar of the local authorities for years before the July 29 knife attack in Southport, a town north of Liverpool.

After the attack, Britain was rattled by a series of riots as disinformation about the attacker’s identity swirled on social media and messaging apps. False claims that he was an undocumented immigrant or newly arrived asylum seeker were amplified by far-right agitators. Mr. Rudakubana is a British citizen who was born in Wales to parents originally from Rwanda.

At age 13 and 14, he was referred three times to Prevent, a British counterterrorism program, because of his fixation on violence, but those referrals were ultimately dropped because it was determined each time that he did not meet the threshold for intervention.

Prime Minister Keir Starmer said from Downing Street on Tuesday that the attack was a sign that terrorism in the country was evolving, and that young people were being radicalized by “a tidal wave of violence freely available online.”