An internal dispute among investigators over leaks to the news media is hampering the major inter-agency probe of the Boston Marathon bombing, according to government sources close to the investigation.

In a major development, the FBI on Thursday made public images and surveillance video of two suspects in the blast and asked for the public’s help in locating them. The men appeared to be in their 20s and were wearing coats and carrying back packs.

Richard DesLauriers, special agent in charge in Boston, said one man, wearing a white cap, was seen placing one of the two devices that exploded.

"At this time, these are the people of interest to the FBI," DesLauriers told reporters. "We consider them to be extremely dangerous and armed."

The man who planted one of the bombs placed it along the marathon route "minutes" before it exploded inside a pressure cooker-based homemade bomb, he said. The blast killed three people and injured 170 others on Monday along the final leg of the marathon route.

Investigators are looking into whether al Qaeda or radical Muslim terrorists were behind the bombings, based on an al Qaeda magazine published in 2010 showing how to construct a bomb inside a pressure cooker that would not easily be detected by bomb-sniffing dogs.

DesLauriers was asked if there are any concerns that additional bombings could be carried out. "There is no additional imminent danger that we are aware of right now," he said.

However, one source close to the investigation said security agencies nationwide remain on heightened alert and the Israeli government also has taken steps to tighten security at diplomatic and other facilities in the Northeast.

The internal dispute surfaced Wednesday as a result of the major break in the case: the discovery of video identification of first one, and then a second, suspect believed to be behind the pressure-cooker improvised bombs.

According to the sources, the FBI has clamped down on all information gathering in the fast-moving investigation.

The investigators want to avoid a repeat of the erroneous cable television news reports Wednesday that a suspect in the bombing had been arrested.

The Associated Press and later CNN, quoting federal and local police sources, first reported that an arrest was made, only to be forced later to issue an embarrassing correction that the reports were false. Other news outlets followed the CNN report.

One official source said the dispute centered on who would be the first to release information to the public that a suspect had been discovered.

In past major crime investigations involving federal and local police similar disputes have broken out but the differences are normally kept from the public.

The official said investigators are very worried that the media disclosures undermined efforts to locate the suspect or suspects, by tipping them off and perhaps allowing them to don disguises and escape the national and international manhunt now underway.

FBI and Boston police spokesmen could not be reached for comment on the internal dispute.

FBI officials, including the lead investigator Richard DesLauriers, are now seeking to better control information gathered from the investigation.

In other developments, Homeland Security Secretary Janet Napolitano told a House Homeland Security Committee hearing that the FBI obtained video near the finish line of the marathon that identified people being sought in the bombing probe.

"I wouldn't characterize them as suspects under the technical term, but we need the public's help in locating these individuals," she said.

"The investigation is proceeding apace," Napolitano said.

Investigators have recovered parts of the pressure cooker and fragments of electronic components, which are now being examined at the FBI forensic laboratory in Virginia.

President Barack Obama visited Boston on Thursday to take part in an interfaith service. He praised the firefighters and emergency response personnel who came to the aid of bomb victims.

The president also visited victims and their families at Massachusetts General Hospital.

The Boston Globe reported that a battery used to detonate at least one of the bombs was the Tenergy Sub-C battery used for remote-controlled model cars.

The pressure cooker identified as carrying one of the bombs was made by Fagor America Inc. The company said in a statement Wednesday that it was contacted by investigators and is cooperating, the Globe reported.

Fox News reported that investigators are obtaining leads in the investigation from social media. More than 30,000 social media messages were gathered and traced to a one-mile radius of the area around the explosions.

The investigators are using "link analysis" to trace relationships and look for any links to the blasts.