London's police chief says the bomb that failed to fully detonate last week in a London subway car was loaded with explosives and shrapnel.

Metropolitan Police Commissioner Cressida Dick told LBC radio on Friday that the bomb discovered last week was "very dangerous" and could have been "much worse."

Thirty people were injured when the homemade bomb partly detonated on a train stopped at Parsons Green station in southwest London.

Authorities say two of the six men arrested over the bombing were released Thursday without being charged. Four others remain in custody.

One of those released is a 21-year-old, originally from Syria, detained outside a fast-food restaurant in west London on Saturday. The Syrian refugee's employer says the man's mother suffered a heart attack after hearing of her son's arrest.