Mark Arsenault and Shelley Murphy report in the Boston Globe investigators combing through the aftermath of the Boston Marathon bombings have found evidence that timing devices were used to detonate the bombs that exploded on Boylston Street, near the race finishing line, Mon., Apr. 15. It has also ­been determined the two bombs were probably fashioned from 6-liter pressure cookers, filled with nails and small ball bearings and then hidden in black nylon bags or backpacks and left on the ground. The two explosions, timed 12 seconds apart, killed three people and injured 176 others. More details were ­revealed Tuesday, on the three people who died — an 8-year-old boy from Dorchester, a 29-year-old woman from Medford, and a Chinese graduate student at Boston University. The FBI is leading the investigation. It has asked the public for tips on anyone who may have been lugging a heavy black bag near the finish line. The White House confirmed President Barack Obama would visit the city Thursday, to speak at an interfaith prayer service. Read the full story in the Boston Globe.