In a potentially vital first breakthrough, investigators were said to have identified a suspect in Monday's double bombing of the Boston Marathon that killed three people and wounded more than 170 others in America's bloodiest terrorist incident since the 9/11 attacks.

A law enforcement source said officials had "made a clear identification of a potential suspect" using CCTV video footage taken from the Lord & Taylor department store near the location of the second explosion, according to CNN.

An image of a potential suspect "carrying, and perhaps dropping, a black bag" at the scene of the second bombing on Boylston Street had been obtained by authorities, the Boston Globe reported. The development came as federal and local investigators pored over the twisted fragments of a pressure cooker, shards of metal shrapnel, shreds of nylon, and thousands upon thousands of photos and video clips, to track down the author of the attacks.

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Simultaneously, a picture was emerging of the two fairly primitive devices – widely sold pressure cookers, loaded with gunpowder, nails, ball-bearings and shrapnel, and detonated by simple timers, rather than by remote control.

The home-made bombs appear to have been carried in nylon backpacks or bags, which were left along the closing stretch of the race.

As news of the CCTV footage emerged, internet users began attempting to take the law into their own hands, with a group posting a 57-picture album on website imgur.com featuring people wearing black backpacks.

On social news site Reddit, one group had more than 100 threads speculating on CCTV images. However, one user cautioned against users taking matters into their own hands.

"We do not condone vigilante justice. Our aim is simply to provide tips for the FBI, not to take matters into our own hands," a user known as Rather_Confused said in a posting. "While it’s admirable to help, posting information on anyone noted in pictures could have a devastating effect on their life."

Adding to the tension across the nation, the Secret Service in Washington announced that a letter to Barack Obama containing a "suspicious substance" had been intercepted at an off-site screening centre where mail to the White House is routinely inspected.

This letter, according to early media reports, was "similar" to one containing traces of the poison ricin which arrived on Tuesday at the Capitol Hill offices of Mississippi's Republican Senator Roger Wicker, and possibly other senators as well. In response, Congressional police briefly cleared parts of two Senate buildings over concerns over suspicious letters and a package.

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Later, Paul Curtis, 45, was arrested at his flat in connection with the packages. Authorities however stressed there was no indication that any of these developments in the capital were connected with the Boston outrage.

Nonetheless, the sequence of events is eerily reminiscent of 12 years ago when, as the country reeled from the attacks on the World Trade Centre and the Pentagon, items of mail containing anthrax spores – far deadlier than ricin – were addressed to two Democratic senators and various media offices. Across the political heart of Washington, security precautions have been heightened, reflecting the current jittery mood.

But the main focus remains on Boston, where the FBI released pictures of a ripped backpack and the remains of a pressure cooker saucepan which it believes were used in the explosions. Today, the lid of a pressure cooker was discovered on the roof of a building close to the finishing stretch of the race along Boylston Street, one of the busiest east-west thoroughfares in downtown Boston.

The hope is that the pictures will jog memories, particularly at the stores where the items might have been purchased, or among people who might have seen a neighbour acting, in retrospect, suspiciously. Despite the hints of major progress, investigators warn that the probe could yet be long and painstaking.

All the items believed to have been used for the bombings are very easily obtainable. The pressure cooker model, for instance, is apparently made by a Spanish company, which sells some 50,000 of them annually in the US. They are reportedly stocked by no less than nine retailers within a mile of the marathon finish line.

In a separate part of the intricate jigsaw puzzle, the FBI and police have been combing through vast numbers of photos and video clips of the bomb scene, before and after the explosions, using state-of-the-art facial recognition technology. "We will go through every frame of every video to determine who exactly was in the area," said Boston police commissioner Edward Davis. On the day of the race, he added, "this was probably one of the most photographed areas in the entire country".

Today a near mile-long stretch of Boylston Street and adjacent blocks were still closed off, 48 hours after the two blasts, that authorities believe were deliberately planned to kill and maim as many people as possible.

According to a July 2010 intelligence report by the FBI and the Homeland Security Department, pressure-cooker explosives have been employed in Afghanistan, India, Nepal and Pakistan, while one of the three devices used in an attempted bombing in Times Square in 2010 used a pressure cooker. The technique moreover has been promoted by al-Qa'ida's branch in Yemen, which gave a detailed description of how to make a pressure cooker bomb in a 2010 issue of Inspire, its English-language online publication, targeted at aspiring terrorists who act alone.

"We don't know whether it was planned and executed by a terrorist organisation, foreign or domestic, or was the act of a malevolent individual," Mr Obama said. He is due to fly into Boston today for an interfaith service.

Meanwhile Iran, the US's fiercest critic in the Islamic world, condemned the Boston bombings, yet criticised the US for employing a double standard over drone attacks that kill innocent civilians. The Islamic Republic "is opposed to any bombings and killings of innocent people no matter if it is in Boston, Pakistan, Afghanistan, Iraq or Syria and condemns it," Ayatollah Ali Khamenei told Iranian military leaders in Tehran. But he attacked the Obama administration for killing people with drones in Pakistan and Afghanistan. "What kind of logic is it that if children and women are killed by Americans in Afghanistan and Pakistan… it is is not a problem, but if a bombing happens in the US or another Western country, the whole world should pay the cost?"