The US city of Boston has been locked down as a massive manhunt for the second Boston Marathon bombing suspect, identified as Dzhokhar Tsarnaev, 19, continues.The US police identified the surviving bombing suspect as of Russian origin, asking people to remain indoors after describing Dzhokhar as armed and dangerous. Local media reports said on Friday that the 19-year-old man lived in Cambridge, Massachusetts. According to the suspects' uncle, both men have lived in US for about a decade. The other suspect, who was killed in a shootout with police, is said to be Dzhokhar’s brother. The 26-year-old man was identified as Tamerlan and died in a shootout in Watertown. The shootout took place when police were looking for the bombing suspects and the assailant who killed a police officer at the Massachusetts Institute of Technology (MIT). Two bombings in Boston on Monday left three people dead and more than 170 injured. Police believe there is a link between the blasts at the Marathon and the shooting incident on the MIT campus. The United States has experienced several mass killings over the past six months including an incident on December 14, 2012, when a 20-year-old man, identified as Adam Lanza, killed twenty children and six adults before pointing the gun at himself at Sandy Hook Elementary School in Newtown, Connecticut.