Police have nabbed a second man in connection with Friday’s London subway attack.

The 21-year-old suspect was collared in Hounslow in West London on Saturday night, and investigators were searching a home in Surrey, about four miles away, that he is associated with, authorities said.

Earlier Saturday, police arrested an 18-year-old man about an hour away in Dover.

The younger man is believed to have lived in a foster home with a married couple who took in Syrian refugees and were once honored for their charitable work by Queen Elizabeth, reports said.

A neighbor, Serena Barber, 47, told the Telegraph that police had confronted the unruly 18-year-old within the past two weeks, and foster parents Penny and Ron Jones were at their wits’ end over how to deal with him.

“He kept getting in trouble with the police and they didn’t know what to do with him,” Barber said.

Police would not comment on whether the teen was known to them, the Telegraph reported.

Another resident told The Sun that she once asked the teen “if he wanted to come to my church to help settle in and meet people, but he didn’t want to.

“His family [was] not happy he [was] with a Christian family,’’ the unnamed neighbor said.

Investigators believe that the young men have some connection to the attack, in which a crude, homemade device exploded in a London subway train, sending a fireball through the cars and sending commuters fleeing.

Thirty people were injured.

Neither man has been charged with a crime, but police can hold them for up to two weeks if they can prove investigations are gaining ground, the BBC reported.

The Islamic State claimed responsibility for the attack, but investigators said Sunday that they cannot definitively connect the attackers to the radical group.

“It is inevitable that so-called Islamic State, or Daesh, will reach in and try to claim responsibility. We have no evidence to suggest that yet,” Home Secretary Amber Rudd said.

Officials believe that the bomb may have detonated incompletely and by accident.

The country’s terror-threat level has been lowered to “severe” from “critical,” Rudd said, meaning authorities no longer believe another attack is imminent, although they still think one is “highly likely.”

Rudd urged “everybody to continue to be vigilant but not alarmed.”

With Wires