LONDON — A Syrian refugee arrested in connection with the recent London subway attack was released without charge on Thursday, the British police said.

No further action is due to be taken against the man, Yahyah Farroukh, 21, who was arrested by an undercover police squad late Saturday night outside the chicken shop in West London where he worked.

Five men remain in custody on suspicion of involvement in the explosion, including a 17-year-old arrested early Thursday in a South London suburb.

More than 30 people were hospitalized on Sept. 15 after an improvised bomb exploded on a packed subway train at the Parsons Green station in West London — an attack that revived memories of the 2005 attacks on mass transit in London that killed more than 50 people.