US President Barack Obama vowed Thursday that the "evil" Boston marathon bombers would be found as investigators focused their efforts on two potential suspects.
"Yes, we will find you, and yes, you will face justice," Obama told a special service in the city, three days after the two bombs killed three people and injured about 180 in a hail of nails and ball bearings.
"We will find you, we will hold you accountable," he told a congregation of 2,000 at Boston's Cathedral of the Holy Cross in a special visit to show national solidarity after Monday's attack.
"If they sought to intimidate us, to terrorize us," Obama said, then "it should be pretty clear by now that they picked the wrong city to do it."
Obama, who also met relatives of the dead and recovering victims at Massachusetts General Hospital, said Americans had seen "the face of evil" in the attacks.
He listened attentively to speeches by the city's religious leaders at the service.
Nasser Wedaddy, head of the New England Interfaith Council, spoke for American Muslims and highlighted how the Koran says that killing one person "is like killing all mankind."
Wedaddy told how he experienced a car bomb while living in Damascus as a child. "What happened on Monday has shocked and horrified us, but it has also brought us together," he said in a message carefully prepared by Muslim leaders who fear a backlash if the attackers are found to be militant Islamists.
The archbishop of Boston, Cardinal Sean Patrick O'Malley, read a message from Pope Francis in which he said the people of the city should keep "working together to build an even more just, free and secure society."
Acclaimed cellist Yo-Yo Ma played with a choir of teenagers, some of whom fought back tears as they sang for the service.
Obama has vowed a relentless hunt for the attackers and authorities say they want to speak to individuals seen in surveillance camera images around the marathon finish line that was devastated by the pressure cooker bombs.
"There is some video that has raised the question of those that the FBI would like to speak with," Homeland Security Secretary Janet Napolitano told a Congressional hearing. But she added that she would not call the "individuals" in the images suspects.
No claim of responsibility and no arrests have been made in connection with the worst attack in the United States since the September 11, 2001 atrocities.
But media reports said authorities would later release images of two men who could have planted the bombs that sprayed nails, ball bearings and other metal fragments into the crowds.
"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.
The Federal Bureau of Investigation says it has launched a "worldwide" hunt. But the FBI and political leaders have appealed for patience over the pace of the investigation.
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.