LONDON — Britain on Sunday lowered its terrorism threat level, a day after the police arrested a second man in connection with the bombing in a London subway station that left dozens of people injured.

The man, 21, was arrested late Saturday in Hounslow, a borough in West London, the Metropolitan Police said in a statement. The suspect, who was not identified, was detained under the Terrorism Act and was being held at a police station in South London, according to the statement.

The second arrest came the same day the police detained an 18-year-old man “on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism” in connection with the bombing.

That teenager was arrested in the southeastern county of Kent, in the departure area of the Port of Dover, where ferries leave for France, according to The Associated Press.