MANCHESTER, England — British police and intelligence agencies arrested three more suspects 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.

A policeman stands outside a residential property in south Manchester, Britain, May 24, 2017.  [Photo/Agencies]

Home Secretary Amber Rudd said the bomber, identified as British-born Libyan Salman Abedi, "likely" did not act alone when he killed 22 people and wounded dozens at an Ariana Grande concert Monday night in Manchester. She said he had been known to security forces "up to a point." Officials are examining Abedi's trips to Libya as they worked to piece together his allegiances and foil any new potential threats.

Police said three men were arrested 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 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 canceled Wednesday so police officers can be re-deployed, Britain's defense 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 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. 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.

Floral tributes are left for the victims of an attack on concert goers at Manchester Arena, in St Ann's Square, in Manchester, Britain May 24, 2017.  [Photo by Wang Bo/chinadaily.com.cn]

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.

France's interior minister said Abedi is believed to have traveled to Syria and had "proven" links with the Islamic State group.

Minister Gerard Collomb said Wednesday on BFM television that British and French intelligence have information that Abedi had been to Syria. He did not provide details, and said it's unclear whether Abedi was part of a larger network of attackers.

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

Rudd said Britain's increased official threat level will remain at "critical" as the investigation proceeds and won't be lowered until security services are convinced there is no active plot in place.

She also complained about US officials leaking sensitive information about Abedi to the US press.

"I have been very clear with our friends that that should not happen again," she said.

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.

He said some of the reported 59 wounded had been discharged, but that the number of patients being treated had increased due to "walking wounded" who came in hours after the attack. Rouse said many of those hospitalized had serious wounds that would require "very long term care and support in terms of their recovery."Officials said all those hospitalized had been identified.

Soldiers were replacing armed police on Wednesday at sites like Buckingham Palace, 10 Downing Street and Parliament. London Police Commander Jane Connors said the goal is to "make our city as hostile an environment as possible for terrorists to plan and operate."She said armed police will also be ready to respond quickly to any incidents and their walking patrols had been increased.

Collomb, who spoke with May after the attack, said the two countries should continue cooperating closely on counterterrorism efforts despite Britain's pending exit from the European Union.

Katz reported from London. Sylvie Hui in London, Rob Harris in Manchester and Angela Charlton in Paris contributed.

Agencies