British police say they have arrested three more people in connection with the investigation into Monday's deadly bombing after a concert in Manchester.
A statement Wednesday from Greater Manchester Police said only that the arrests came after officers executed warrants in South Manchester. There was no information about how the three men, or a fourth man arrested Tuesday who was only identified only as a 23-year-old, might be involved in the attack.
British interior minister Amber Rudd said Wednesday the suicide bomber, who has been identified as 22-year-old Salman Abedi, was "known" by British intelligence services before the bombing.
She also told the BBC that "it seems likely, possible, that he wasn't doing this on his own."
Investigators have been working since Monday's attack to determine if the bomber was part of a wider group.
Mark Rowley, who heads the National Counter Terrorism Policing, described the investigation late Tuesday as "making good progress" and following a number of leads. But he said authorities cannot yet say if the Abedi acted alone.
His comments came after a panel that sets the country's terrorism alert level raised it to critical, or the highest step, signaling that another attack was highly likely and could be imminent.
The change is most visible in the deployment of soldiers to help guard certain areas, including major events such as concerts and football matches, in order to free up police officers.
The blast at the conclusion of a concert by American pop star Ariana Grande at Manchester Arena killed 22 people and wounded 59 others. The attacker also died at the site.
Islamic State is claiming it was behind the attack, but neither British nor U.S. intelligence have confirmed that.
Bomber likely traveled to Syria
French Interior Minister Gerard Collomb told BFM television on Wednesday that British and French intelligence have information that Abedi had likely traveled to Syria.
British Prime Minister Theresa May said in an address to the nation late Tuesday that authorities will do everything possible to protect the public and asked people to remain vigilant.
"I do not want the public to feel unduly alarmed. We have faced a serious terrorist threat in our country for many years," May said.
Earlier in the day, she spoke more directly about the attack itself, saying the bomber chose the "time and place to cause maximum carnage and to kill and injure indiscriminately."
Many of the victims were young girls, with the youngest identified so far being just 8 years old.
Grande wrote on Twitter that she is, "Broken. From the bottom of my heart, I am so, so sorry. I don't have words."
broken. from the bottom of my heart, i am so so sorry. i don%27t have words.— Ariana Grande (@ArianaGrande) May 23, 2017
In Photos: Manchester suicide bombingView full gallery
Video from the arena showed the joy in the audience at the end of the concert turning to confusion and then to panic and a scramble to get out of the building as the realization of what just happened spread.
Witness say they saw blood covered bodies on the floor while others, badly wounded, staggered toward the exits of the building.
The scene outside the concert hall was also chaotic, with traffic snarled and parents rushing to the scene.
Moment of silence
Britain's Queen Elizabeth held a moment of silence at a garden party at Buckingham Palace. French President Emmanuel Macron signed a condolence book at the British embassy in Paris. German Chancellor Angela Merkel said the attack only strengthens Germany's resolve to work with the British.
The U.N. Security Council also condemned the bombing.
U.S. President Donald Trump, visiting Bethlehem in the West Bank, called those responsible for the blast "evil losers in life."
"I won't call them monsters because they would like that term; they would think that's a great name," Trump said. "I will call them from now on losers, because that's what they are."
He added, "We cannot stand a moment longer for the slaughter of innocent people."
WATCH: Henry Ridgwell reports from Manchester