British authorities have detained a third suspect in connection with Friday's bomb attack on London's Underground subway system, the Metropolitan Police announced Tuesday.

According to a statement, the 25-year-old man was arrested in the city of Newport, in southeast Wales. The Met Police also said an address in the same city was being searched.

"This continues to be a fast-moving investigation," said Met Police Cmdr. Dean Haydon, the head of the Met Counter Terrorism Command. "We now have three men in custody and searches are continuing at four addresses. Detectives are carrying out extensive inquiries to determine the full facts behind the attack."

An 18-year-old man was detained Saturday at the southeast England port of Dover, a departure point for ferries to France. A 21-year-old was arrested later the same day in Hounslow in west London. Photos published by the Sun newspaper showed a man being detained outside a fast-food restaurant in Hounslow, which was searched by police.

Both detainees were reported to be Middle Eastern refugees who had been fostered by of Penelope and Ronald Jones. The Joneses, who live in the London suburb of Sunbury, had been honored by Queen Elizabeth II for fostering more than 200 children, including refugees from Middle Eastern conflicts.

It was not immediately clear if the third detainee had any connections to the couple.

Thirty people were injured when an improvised explosive device partly exploded aboard a crowded London Underground train at Parsons Green station during the morning rush hour. None of the injuries was life-threatening, and experts said it appears the main charge of the bomb didn't detonate.

The ISIS terror group claimed responsibility for the attack, but authorities say there is no evidence of their involvement.

"We are asking the public to look out for anything that seems out of place, unusual or just doesn't seem to fit in with day-to-day life,"Haydon said. "It may be nothing, but if you see or hear anything that could be terrorist-related trust your instincts, then ACT, and call us."

The Associated Press contributed to this report.