Investigators have released images of two men who were at the sites where twin bombs were detonated at the Boston Marathon.
The FBI is asking for the public's help in identifying the men. A web site has been set up for tips: https://bostonmarathontips.fbi.gov/
Anybody with information is asked to call 1-800-CALL-FBI.
The official said investigators want to question the men in the bombings - which killed three and injured 176 near the finish line of the Marathon Monday afternoon. The official cautioned that it is unclear whether they are suspects or merely witnesses.
Photos: Boston Marathon attack suspects
At one of the bomb sites, several law enforcement officials told USA TODAY, surveillance video shows a man putting down a bag. The bag appears to be similar to the black nylon pack described earlier as possibly used to carry the explosive devices, the officials said.
The blasts killed 8-year-old Martin Richard of Dorchester, 29-year-old Krystle Campbell of Medford, and Lu Lingzi, a 23-year-old Boston University graduate student from China.
At a service for victims in Boston Thursday, President Barack Obama vowed the attackers would be caught and prosecuted. "Yes, we will find you -- and, yes, you will face justice," Obama said.
In Washington, Attorney General Eric Holder said Thursday that the "full resources'' of the Justice Department were being deployed in the investigation and to guard against any future attacks.
Holder, the nation's top law enforcement officer, told a Congressional committee Thursday that the "individual or group that carried out this heinous act'' would be held accountable... "by any means available to us"
Holder's comments came after Homeland Security Secretary Janet Napolitano told a House panel that the responsibility for the attack has yet to be linked to individuals, international or domestic terror organizations.
"We don't know,'' Napolitano told a congressional panel, as investigators in Boston continue to pursue a number of promising leads.
Authorities continue working to identify individuals at both sites as well as a number of other people in video and photographs near the explosions.
More than 50 people remain hospitalized, with 7 listed in critical condition, down from 14 Wednesday. They include a 10-year-old boy at Boston Children's Hospital whose leg was amputated and a nine-year-old girl with a leg injury. Brigham and Women's Hospital had four people in critical condition and Boston Medical Center had one. At Tufts Medical Center, five bombing victims remain under care, but none are listed as critical.
Meanwhile, families of a newlywed couple who both suffered a leg amputation below the knee are thanking supporters for an outpouring of donations.
The families of Patrick and Jessica Downes said Thursday that the generosity from friends and strangers has restored their faith in humanity. Over 4,400 people have donated more than $246,000 to the couple through GiveForward.com.
The families say they don't want the couple to worry about paying for prosthetics and modifications to their home during the recovery process.
Thirty-year-old Patrick and 32-year-old Jessica are in stable condition and recovering from surgery. They were married in Boston in August. Jessica is a nurse at Massachusetts General Hospital, one of several area hospitals treating bomb victims.
Contributing: G. Jeffrey MacDonald; the Associated Press