The foreigner, also badly injured in the blasts, was in the US on a student visa and is considered a person of interest and has not been formally charged or held. The potential suspect is currently undergoing treatment in a hospital.
Some news reports said the foreigner was a Saudi Arabian national although it was not officially confirmed. Boston Police Commissioner Ed Davis said there was no suspects as such at Brigham and Women's Hospital although he acknowledged that there were people with whom they were speaking to.
Law enforcement officials said the 20-year-old injured student acted suspiciously when the explosions took place along the marathon route. The man was caught two hours after the bombing on the finish line of the marathon, a US media source reported. The man will also undergo physical and forensic test, authorities said.
President Barack Obama on Monday said that whoever had carried out the ghastly crime would be brought to justice but asked people not to jump to conclusions before facts were gathered. Obama addressed the nation on Monday evening after the law enforcement authorities tightened security at the White House and in Washington.