BOSTON — Investigators in Boston focused Thursday on video images of two potential suspects, one of whom was recorded dropping a black bag near one of the blast sites.

BOSTON — Investigators in Boston focused Thursday on video images of two potential suspects, one of whom was recorded dropping a black bag near one of the blast sites.

Images collected by the FBI show a man dropping a bag near the scene of one of the two explosions in the attack, according to federal law-enforcement officials. A second man is also a potential suspect. Scraps of nylon recovered from the blast site indicate at least one bomb was concealed in a black bag.

The investigation involving more than 1,000 law-enforcement personnel continued as President Barack Obama attended a memorial for the victims of the bombing. Addressing the hundreds of citizens at the Cathedral of the Holy Cross, Obama said the perpetrators will be caught.

“Yes, we will find you, and yes, you will face justice,” he promised.

Congregants also included first lady Michelle Obama and dozens of police officers and emergency personnel, medical workers dressed in scrubs and Boston Marathon participants in their running gear. Obama’s 2012 Republican opponent, former Massachusetts governor Mitt Romney, was among the dignitaries.

The April 15 attack killed three people and wounded scores in the highest-profile act of terror in the United States since the 2001 attacks on New York City and Washington.

The FBI has identified people from videos and still pictures whom authorities want to interview, according to a person briefed on the investigation Wednesdaynight.

The pictures have come from several vantage points, including store security cameras on and across the street from the bomb site, according to federal law-enforcement officials.

The city at the center of the nation’s 10th-biggest metropolitan area went about its business Thursday armored by a new apparatus of security. At the Sheraton Boston Hotel, near the bombing site, conventioneers mingled with guards in camouflage carrying automatic weapons. Morning commuters waiting for the Silver Line bus in the South End watched a white truck from the Bureau of Alcohol, Tobacco, Firearms and Explosives cruise through the neighborhood.

Homeland Security Secretary Janet Napolitano, testifying Thursday morning before the House Homeland Security Committee, said video from the marathon raises questions about some individuals. She added that she “wouldn’t characterize them as suspects.”

The Boston Police Department had “nothing new” to report, said Angeline Richardson, a spokeswoman.