Last night United Kingdom prime minister Theresa May raised the terror threat in Britain to "Critical", the highest it has been for a decade, indicating a terror attack is expected imminently.
Manchester police suspect that the bomber, 22-year-old Salman Abedi, may not have acted alone.
The Polish Foreign Ministry, which did not name the couple, said: "We share the pain of the family and the loved ones of the victims and express our most honest condolences".
British authorities were probing whether Abedi had ties to other cells across Europe and North Africa, according to two officials familiar with the case who spoke on condition of anonymity because they were not authorized to speak about the ongoing investigation.
The identities of the victims were becoming known little by little.
Omar Alfa Khuri, who lives across the street, said he was awakened at 02:30 by a loud noise and saw police take away the father of the family that lives there in handcuffs. The daughters were safe.
The attack killed 22 people and injured more than 100. "We're dealing with injuries to major organs, we're dealing with loss of limbs potentially, we're dealing with embedded objects", Jon Rouse, chief officer of the Greater Manchester Health and Social Care Partnership told the BBC.More news: Brad Pitt Displays A Stunning Look In Tokyo, While Promoting War Machine
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Even though the senior Abedi denied that he was a member of the Libyan Islamic Fighting group, former Libyan security official Abdel-Basit Haroun told the AP on Wednesday that the elder Abedi was a member in the 1990s of the group, which had links to al-Qaida.
"I mean the British police have been very clear that they want to control the flow of information in order to protect operational integrity, the element of surprise, so it is irritating if it gets released from other sources and I have been very clear with our friends that should not happen again".
His remarks go a step further than previous statements, in which officials said they were trying to learn whether the attacker had the support of a larger group.
"It is a possibility that we can not ignore that there is a wider group of individuals linked to this attack". The arrests come after Prime Minister Theresa May raised the terror threat level to "critical", the highest setting. As part of the security response, military personnel are being deployed into civilian areas to work alongside and bolster police.
Attacks in cities including Paris, Nice, Brussels, St. Petersburg, Berlin and London have shocked Europeans already anxious over security challenges from mass immigration and pockets of domestic Islamic radicalism.
Business Insider reported on Tuesday how several key details on the attacker's identity had made their way to U.S. news channels - potentially compromising the ongoing investigation.