President Barack Obama has ordered U.S. flags to be flown at half-staff as a mark of respect for the victims of the Boston Marathon bombing.
The order directs U.S. flags to be flown at half-staff at all public buildings and grounds, all military posts and naval stations until sunset on Saturday, April 20.
Governor Tom Corbett later joined the nationwide tribute and asked that Pennsylvania flags be lowered to half-staff.
Three people were killed, including an 8-year-old boy, by two explosions Monday just seconds apart near the finish line.
Boston police said 176 victims were treated at hospitals around Boston, and 17 of those were in critical condition.
Authorities are looking for amateur video and photographic evidence that can give clues to who set off the bombs.
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