Investigators of the deadly Boston Marathon bombing have an image from surveillance video of a man dropping off a bag containing one of the bombs, a politician said Wednesday.

City Council President Stephen Murphy said he didn't know if investigators have identified the man, who was seen on surveillance footage from a nearby department store. Murphy said police involved in the probe say investigators have matched information from the footage with witness descriptions of someone leaving the scene.

“They may be on the verge of arresting someone and that's good,” Murphy said.

Monday's twin blasts killed three people and wounded more than 170. The dead included a child, a young woman and a female student from China.

A law enforcement official said investigators have an image of a potential suspect but do not know his name and have not questioned him. The official said investigators made the discovery while looking over photos and video of the scene. The official was not authorized to discuss the case publicly and spoke to The Associated Press on condition of anonymity.

Police and reporters converged on the federal courthouse in a jittery city amid reports of a breakthrough in the investigation and conflicting information on whether a suspect was in custody. Several media outlets reported that a suspect had been identified from surveillance video taken at a Lord & Taylor department store between the sites of the two bomb blasts.

President Barack Obama called the attack on the world's most famous marathon an act of terrorism. Obama planned to attend an interfaith service Thursday in the victims' honor in Boston.

A news briefing was expected later Wednesday. A bomb threat forced the evacuation of the courthouse in midafternoon, the U.S. Marshals Service said, but workers were allowed back into the courthouse a short time later.

A law enforcement official briefed on the investigation told The Associated Press earlier in the day that a suspect was in custody. But the FBI and the U.S. attorney's office in Boston said no arrests had been made. The official who spoke to the AP did so on condition of anonymity and stood by the information even after it was disputed. The official was not authorized to divulge details of the investigation.

“Contrary to widespread reporting, there have been no arrests made in connection with the Boston Marathon attack,” the FBI said in a statement. “Over the past day and a half, there have been a number of press reports based on information from unofficial sources that has been inaccurate. Since these stories often have unintended consequences, we ask the media, particularly at this early stage of the investigation, to exercise caution and attempt to verify information through appropriate official channels before reporting.”

Law enforcement agencies had pleaded for the public to come forward with photos, videos or any information that might help them solve the twin bombings near the race's finish line. The bombs exploded 10 or more seconds apart just next to the race course, tearing off victims' limbs and spattering streets with blood. The blasts went off minutes after the four-hour mark of the race, a high-traffic time when thousands of runners pour toward the finish.

The bombs involved kitchen pressure cookers packed with explosives, nails and other lethal shrapnel. But the FBI said nobody had claimed responsibility.

An intelligence bulletin issued to law enforcement includes a picture of a mangled pressure cooker and a torn black bag that the FBI said were part of a bomb that exploded during the marathon.

Authorities recovered a piece of circuit board that they believe was part of one of the explosive devices, and they found the lid of a pressure cooker that apparently was catapulted onto the roof of a nearby building, a law enforcement official said Wednesday. The official spoke on condition of anonymity because this person was not authorized to publicly discuss evidence in the ongoing investigation.

The blasts killed 8-year-old Martin Richard and 29-year-old Krystle Campbell. The Shenyang Evening News, a state-run Chinese newspaper, identified the third victim as Lu Lingzi. She was a graduate student at Boston University.

Scores of victims remained in hospitals, many with grievous injuries. A 5-year-old child, a 9-year-old girl and 10-year-old boy were among 17 victims listed in critical condition.

Officials at three Boston hospitals that treated some of the most seriously injured said they expect all of their patients to survive.

The trauma surgery chief at Boston Medical Center said most of the injuries his hospital treated were to the legs.

“We have a lot of lower extremity injuries, so I think the damage was low to the ground and wasn't up,” Dr. Peter Burke said.

At Massachusetts General Hospital, all four amputations performed there were above the knee, with no hope of saving more of the legs, said Dr. George Velmahos, chief of trauma surgery.

“It wasn't a hard decision to make,” he said Tuesday. “We just completed the ugly job that the bomb did.”

Pressure-cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 intelligence report by the FBI and the Homeland Security Department. One of the three devices used in the May 2010 Times Square attempted bombing in New York City was a pressure cooker, the report said.

“Placed carefully, such devices provide little or no indication of an impending attack,” the report said.

But information on how to make the bombs is readily found online, and U.S. officials said Americans should not rush to judgment in linking the attack to overseas terrorists. (AP)