Boston and its surburbs, universities and transit system were on total lockdown Friday as police hunted for marathon bombing suspect Dzhokhar Tsarnaev — on the loose after his terrorist brother was killed in a stunning chain of events that left one cop dead and another injured, officials said.

Authorities were confronting a double-edged nightmare: a ruthless killer at large in a densely populated area and a four-mile stretch of road possibly littered with explosive devices tossed from the suspects’ getaway vehicle during a wild chase and firefights.

Two unidentified people were taken into custody at the Cambridge, Mass., home where Dzhokhar Tsarnaev and his older brother Tamerlan grew up, but they were not being described as additional suspects. Three dozen FBI agents were ringing the house.