Eight men are now in custody in connection with Monday's suicide bombing at a Manchester, England, pop concert, including two men who were arrested in separate raids early Thursday.
A woman arrested Wednesday during a raid on an apartment north of Manchester has been released without charge.
Greater Manchester Police Chief Constable Ian Hopkins said these past few days have been "intense" for the officers and staff of the department, but that they continue to make progress in the investigation.
"I want to reassure people that the arrests that we have made are significant. And initial searches of premises have revealed items that we believe are very important to the investigation," Hopkins said Thursday.
Police have given no information about how the detained men may be connected to the bombing.
WATCH: Manchester police chief on latest developments
A moment of silence was held in Manchester Thursday morning for the victims.
Residents gathered in a large circle in the city's St. Ann's Square with heads bowed before a makeshift memorial of flowers, balloons and candles dedicated to the 22 people killed when Salman Abedi detonated an improvised device just moments after a concert by American pop singer Ariana Grande.
Shortly after, Queen Elizabeth arrived at Royal Manchester Children's Hospital to visit some of the 64 people who were injured. Many of the victims were young children.
In a televised message, Prime Minister Theresa May announced that Britain's terror threat level will remain at critical.
On Friday, U.S. Secretary of State Rex Tillerson will make his first official visit to London to meet with his counter-part, Boris Johnson.
During the visit he is expected to "reaffirm America's commitment to the Special Relationship between the United States and the United Kingdom and our solidarity in defeating terrorism in every part of the world," according to a State Department spokeswoman.
Father, brother arrested
Abedi's father and brother were arrested by Libyan security forces Wednesday. A spokesman for the Libyan anti-terrorism force said the brother, Hashim Abedi, had recently been in contact with Salman Abedi and knew of his plans for the attack.
Abedi's father, Ramadan, told Reuters he spoke to his son five days ago, and "everything was normal."
Salman Abedi did not have extremist beliefs, Ramadan Abedi said in Tripoli, where he lives. He added, however, that his son did not disclose he was heading for Manchester when he left Libya last week.
The elder Abedi told the Reuters news agency he condemns "terrorist acts on civilians and innocent people."
According to a spokesman for Libya's anti-terror force, Salman Abedi called his mother just hours prior to the attack to say, "Forgive me."
Special Deterrent Force spokesman Ahmed bin Salem said Thursday the mother told investigators Salman Abedi left Libya for England four days before the bombing took place.
"He was giving farewell," Bin Salem said.
Bomber traveled to Syria
In France, Interior Minister Gerard Collomb said Wednesday that British and French intelligence have information that Abedi had likely traveled to Syria.
Collomb told France's BFM television that Abedi "grew up in Britain and then suddenly, after a trip to Libya and then likely to Syria, became radicalized and decided to carry out this attack. In any case, the links with Daesh are proven," he said, using a term for Islamic State.
The terror group has claimed responsibility, but British and U.S. intelligence have not confirmed that the