Police stand guard near the Manchester Arena on May 23, 2017 in Manchester, England. At least 22 people were killed in a suicide bombing at an Ariana Grande concert at Manchester Arena which was packed with children. It is the worst terrorist incident on British soil since the London bombings of 2005. (Photo by Jeff J Mitchell/Getty Images)

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MANCHESTER, England (AP) — Police in Manchester say they have arrested three more men in connection with the suicide bombing at an Ariana Grande concert that killed 22 people.

Police made the arrests south of the city, where a day earlier a 23-year-old man was also arrested and a number of homes were searched.

GMP statement – Three men arrested pic.twitter.com/ohuGs4iR8g — Greater Manchester Police (@gmpolice) May 24, 2017

Police are trying to establish if bomber Salman Abedi acted alone or whether there could be a risk of further attacks. Investigators believe it’s “likely” that Abedi had help to carry out the attack.

On Tuesday, Britain raised its threat level to “critical” over concerns that Abedi may have accomplices who planned to carry out more attacks. British soldiers have been deployed in place of police officers to guard high-profile sites such as Buckingham Palace and Parliament.

Abedi was born in Britain to a Libyan family, grew up in Manchester’s southern suburbs and attended the local Salford University for a time.

Police on Tuesday raided his house, using a controlled explosion to blast down the door. Neighbors recalled him as a tall, thin young man who often wore traditional Islamic dress and did not talk much.

British Prime Minister Theresa May Wednesday chaired a meeting of her emergency security cabinet group known as Cobra to deal with the intelligence reports about Abedi and concerns that he might have had outside support.

Police also raided and searched a property elsewhere in Manchester where Abedi’s brother Ismail is thought to have lived. A 23-year-old man has also been arrested as part of the investigation but officials have released no details about him.

Officials are probing how often Abedi had traveled to Libya, which has seen an eruption of armed Islamist groups since dictator Moammar Gadhafi was overthrown and killed in 2011.