British police and intelligence agencies arrested three more suspects on Wednesday in connection with the Manchester suicide bombing and moved quickly to secure key sites across the country, including Buckingham Palace and the British Parliament at Westminster.

Police said three men were arrested on Wednesday in south Manchester, where a day earlier a 23-year-old man was also arrested and a number of homes were searched.

Britain raised its threat level from terrorism to “critical” after an emergency government meeting on late Tuesday amid concerns that the 22-year-old Abedi may have accomplices who are planning another attack. British soldiers have been deployed in place of police officers to guard high-profile sites such as Buckingham Palace and Parliament.

The changing of the guard ceremony at Buckingham Palace was cancelled Wednesday so police officers can be re—deployed, Britain’s defence ministry said. The traditional ceremony at the palace is a major tourist attraction in London.

The Palace of Westminster, which houses the British Parliament in London, was also closed on Wednesday to all those without passes, and tours and events there were cancelled until further notice. Armed police were seen on patrol outside St. Paul’s Cathedral in London, another popular tourist spot.

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. Neighbours 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 travelled to Libya, which has seen an eruption of armed Islamist groups since dictator Muamar Qadhafi was overthrown and killed in 2011.

British officials have not commented on whether Abedi had links to IS or other extremist groups.

In addition to those killed in the concert attack, 64 people are being treated for their wounds, Jon Rouse of the Greater Manchester Health and Social Care Partnership said Wednesday.

Officials said all those hospitalized had been identified.