President Barack Obama on Thursday sought to console Boston after the deadly terror attack on the city's marathon as police focused their hunt on two suspects caught on video cameras.
The president and First Lady Michelle Obama attended a special inter-faith service at Boston's Cathedral of the Holy Cross with victims of Monday's double bomb attack, rescuers, and city and state leaders.
While Obama has vowed a relentless hunt for the attackers, he went to Boston with a message of "healing" to help the city past the devastation of the bombs at the race's finish line, which killed three people and wounded 180.
He also plans to meet with some of the victims of the bombings.
No arrests have been made in connection with the twin bombings that are the worst attack in the United States since the September 11, 2001 attacks.
But media reports said authorities would later release images of two suspects believed to have planted the pressure cooker bombs that sprayed nails, ball bearings and other metal fragments into the thousands-strong crowd.
"Authorities have clear video images of two separate suspects in the Boston Marathon bombings carrying black bags at each explosion site and are planning to release the images today," the Boston Globe said, quoting an official.
The images were from surveillance cameras in Boylston Street in Boston where the marathon ended.
A law enforcement official confirmed to AFP on Tuesday that investigators have images of at least one potential suspect and are seeking "to locate and identify that individual."
The suspects have not yet been identified though, reports said.
No claim of responsibility has been made for the attack and the Federal Bureau of Investigation (FBI) says it has launched a "worldwide" hunt.
The FBI and political leaders have appealed for patience over the pace of the investigation however. "They are making progress. But it's going to take time," Massachusetts governor Deval Patrick said.
The FBI released photographs of the mangled metal remnants of a pressure cooker believed to have been used for one of the bombs. The lid of one pressure cooker was found on the roof of a hotel near the marathon finish line.
More than 100 of the injured have left Boston hospitals, but about 10 remain in critical condition and some will require new operations.
At least 12 people have lost at least one of their legs because of the blast from the bombs, which fired the metal fragments at low level.
Boston has held emotional tributes to the dead -- eight-year-old Martin Richard, Boston University graduate student Lu Lingzi of China and restaurant manager Krystle Campbell.
Doctors at Boston Medical Center said a second Chinese student caught in the blast had come out of a coma and was improving. The girl's family was expected in Boston soon.
Obama and other speakers at the cathedral service were expected to pay tribute to the three at the special service.