As the police conducted a series of house raids on Thursday in the Manchester area, the chief investigator said they were making headway in the investigation into the plot and had amassed important evidence. A man was arrested in the Moss Side area early Friday, according the police. Eight suspects were in custody. Two other people were released after being detained.

“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,” Manchester’s top police official, Chief Constable Ian Hopkins, said at a news conference. “These searches will take several days to complete, as you would expect. Therefore, there will be some disruption. However, it is very important that we continue with these searches.”

Officials in Turkey and Germany said on Thursday that Mr. Abedi had passed through airports in Istanbul and the German city of Düsseldorf this month on his way back to Britain from Libya.

It did not appear that he had stayed very long or even left the airports in Turkey and Germany, but the German authorities were looking into a 2015 visit by Mr. Abedi to Frankfurt.