President Barack Obama to visit Boston on Thursday

President Barack Obama will visit Boston on Thursday, three days after a pair of deadly bombings at the Boston Marathon. Massachusetts Gov. Deval Patrick says Obama will be attending an interfaith service.

Obama has canceled a trip planned for Friday to Kansas, the White House said.

Three people were killed at the marathon's finish line, including an 8-year-old boy. More than 870 were wounded.

Hours after the explosions, Obama vowed that those responsible would be brought to justice. In remarks on Tuesday, he called the bombings an act of terrorism but said investigators still don't know who carried them out.

He also said the American people refuse to be terrorized.

Obama has traveled four times to cities reeling from mass violence, but all of the previous trips followed shooting incidents. The most recent was in December after the schoolhouse shooting in Newtown, Conn.

FBI Special Agent in Charge Richard DesLauriers said investigators also had recovered fragments of BBs and nails from the scene of Monday's back-to-back explosions, which killed three people and wounded more than 180 others. The devices may have been constructed from pressure cookers, he said.

Separately, President Barack Obama will come to Boston on Thursday to attend a memorial service for the blast victims, Massachusetts Governor Deval Patrick said.