By JILL LAWLESS, MAGGIE MICHAEL and GREGORY KATZ

(AP) - British security forces raided an apartment building Wednesday in central Manchester as they investigated "a network" of people allegedly behind the city's concert bombing. Hundreds of soldiers were sent to secure key sites across the country, including Buckingham Palace and the British Parliament at Westminster.

Home Secretary Amber Rudd said the bomber, Salman Abedi, "likely" did not act alone when he killed 22 people and wounded scores at an Ariana Grande concert Monday night in Manchester. She said he had been known to security forces "up to a point."

Abedi, a 22-year-old British citizen born to Libyan parents who grew up around Manchester, died in the attack.

Manchester Police Chief Constable said four people have been arrested thus far as police raid properties thought to be connected to Abedi.

"I think it's very clear this is a network we are investigating," he said, adding that an off-duty police officer was among those killed in concert attack.
Many at the concert were young girls and teens enthralled by Grande's pop power - and those who died included an 8-year-old girl.

Officials are examining Abedi's trips to Libya and possibly Syria as they piece together his allegiances and try to foil any new potential threats. The government said nearly 1,000 soldiers were deployed Wednesday in high-profile sites in London and elsewhere, replacing police, who can work on counter-terrorism duties.

Britain raised its threat level from terrorism to "critical" late Tuesday amid concerns that Abedi may have accomplices who are planning another deadly attack.

Police said three suspects were arrested Wednesday in south Manchester.  Another suspect was arrested Tuesday and Abedi's father confirmed to The Associated Press that was Salman's older brother Ismail.

No one has yet been charged in the case and police have not identified the suspects.

Heavily armed police raided an apartment building in Manchester on Wednesday afternoon, blasting the door open with a controlled explosion. Neighbor Adam Prince said the raided apartment had been used as an Airbnb.

Muye Li, a 23-year-old student who lives on the same floor, said he thought officers were looking for a woman because they "asked me if I had seen the lady next door."

Across London, troops fanned out and authorities implemented special security plans.

The changing of the guard ceremony at Buckingham Palace was canceled Wednesday so police officers can be re-deployed. The traditional ceremony is a major tourist attraction in London.

The Palace of Westminster, which houses the British Parliament in London, was closed Wednesday to all those without passes, and tours and events there were cancelled until further notice. Armed police also patrolled outside St. Paul's Cathedral in London.

"(The goal) is to make our city as hostile an environment as possible for terrorists to plan and operate," said London Police Commander Jane Connors.
The Chelsea soccer team announced it would cancel Sunday's victory parade in London to celebrate the team's Premier League title.

> "We are sure our fans will understand this decision," the team said, adding that the parade would have diverted police from the bombing investigation.
Speaking Wednesday from the Libyan city of Tripoli, the father of the alleged Manchester bomber denied that his son was linked to militants or to the deadly attack. Ramadan Abedi told the AP that when he spoke to his son Salman five days ago, he sounded "normal."

He said Salman was getting ready to visit Saudi Arabia for a short Umrah pilgrimage, then planned to head to Libya to spend the Islamic holy month of Ramadan with his family.

"We don't believe in killing innocents. This is not us," the elder Abedi told the AP by telephone. "We aren't the ones who blow up ourselves among innocents. We go to mosques. We recite Quran, but not that."

He said his son last visited Libya a month-and-a-half ago and never visited Syria.

The senior Abedi fled Tripoli in 1993 after Moammar Gadhafi's security authorities issued an arrest warrant for him and eventually sought political asylum in Britain. Now he is a manager for the Central Security force in Tripoli.

He denied having ties to any of Libya's militant groups, including the Libya Islamic Fighting Group, which was linked to al-Qaida.

But former Libyan security official Abdel-Basit Haroun told the AP that the elder Abedi was a member of the Libyan Islamic Fighting group in the 1990s.

Although the LIFG disbanded, Haroun says the father now belongs to the Salafi Jihadi movement, the most extreme sect of Salafism from which al-Qaida and the Islamic State group both hail.

In Manchester, where residents continued to place floral tributes to the dead and injured, police arrested a man at a house near Abedi's home.

Omar Alfakhuri, who lives across the street, said he was awakened at 2:30 a.m. by a loud noise and saw police take away the father of the family that lives there in handcuffs. He said the man, in his 40s, is named Adel and has a wife and several children.

"They arrested the father, and I think the rest of the family kind of disappeared," Khuri said, adding that he knew the man from the neighborhood and the mosque. "In the last 15 years, I haven't seen him in trouble at all."

At Manchester's Didsbury Mosque, where the Abedi family worshipped, authorities condemned the bombing and denied reports that Abedi had worked there. Azhar Mahmoud, who prays at the mosque in south Manchester, said it was "horrible" that the bomber was associated with it.
"Wherever he got that, he didn't get it from this mosque," he said.

Mahmoud said the imam regularly preached against radicalization.

"He always tells the youngsters, stay away from it. ... I've prayed there Fridays, and that's his message."

British Prime Minister Theresa May chaired a meeting Wednesday of her emergency security cabinet group to talk about concerns Abedi might have had outside support. France's interior minister said Abedi is believed to have also traveled to Syria and had "proven" links with the Islamic State group.
Rudd said Britain's increased official threat level will remain at "critical" until security services are convinced there is no active plot in place.
She also complained about U.S. officials leaking sensitive information about Abedi to the press, saying that could hinder Britain's security services and police.

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

Manchester officials raised to 119 the number of people who sought medical treatment after Monday night's attack, saying 20 of them had critical injuries.
Jon Rouse of the Greater Manchester Health and Social Care Partnership said 64 people were still hospitalized, many with serious wounds that will require "very long term care and support."

Officials said all the dead and wounded had been identified but their names would not be made public for several days until autopsies are completed.

Michael reported from Cairo and Katz reported from London. Sylvia Hui and Paisley Dodds in London, Rob Harris in Manchester and Angela Charlton in Paris contributed.