Federal investigators involved in one of the largest manhunts in US history have revealed that the Boston marathon bombs that killed three people and injured more than 170 may have been made from pressure cookers packed with ball bearings and nails.

More than 1,000 law enforcement officials from 30 state and federal agencies have now been committed to tracking down whoever was responsible for Monday's twin blasts, the worst terror attack on US soil since 9/11. The forensic examination of the crime scene has begun to bear fruit, though officials concede that they still have no suspects and no motive for the bombing.

FBI agents leading the investigation are pitting their hopes on the public as they believe the instigator of the attack may well have revealed themselves to others. "The person who did this is someone's friend, co-worker or relative. Someone knows who did this," said Rick DesLauriers, special agent in charge of the Boston FBI, which is leading the investigation.

Forensic examiners have recovered fragments of black nylon cloth they believe to have come from backpacks used to carry both bombs. They have also found what DesLauriers described as "fragments of nails that could have been contained in a pressure cooker". He added that he was talking about a basic household pressure cooker of the sort found in many kitchens.

The devices exploded at about 2.50pm on Monday, within seconds of each other and 50 metres apart, causing a current casualty toll of 176 injured, 17 critically, and three dead. Two of the dead have been named: Martin Richard, an eight-year-old boy from Boston's Dorchester neighbourhood, and Krystle Campbell, 29, who had been at the finish line to watch her boyfriend complete the race. The Chinese consulate in New York said that the third fatal victim was a Chinese citizen whose identity was not being made public at the request of their family.

In a statement from the White House briefing room, President Barack Obama underlined the lack of hard information about what he called the "heinous" acts in Boston, but pointedly made clear the US was now viewing it as "an act of terror".

"We do not know if this was an act of an organisation, an individual or individuals, and we don't have a sense of motive yet," he said. "Everything else is speculation."

Obama will travel to Boston on Thursday to attend a multi-faith service for the victims, the White House said.

As details of the bomb devices began to emerge, the full horror of the incident became clearer. Investigators said the blast sites were massive, with debris found on nearby rooftops and embedded in the walls of buildings.

The remnants of a pressure cooker that the FBI says was part of one of the bombs that exploded during the Boston Marathon. Photograph: FBI/AP

Several of the injured underwent amputations overnight – four at Massachusetts General hospital alone. It was also revealed that the devices had been packed with metal fragments and placed close to the ground, leading to a high number of amputations and serious lower body injuries.

George Velmahos, chief trauma surgeon at Massachusetts General Hospital – where many of the injured were taken – said nails and pellets were removed from the bodies of those hit in the blast.

Detectives clearly think that the heavy nature of the backpacks carried by the bomber or bombers could provide an essential clue that might trigger someone's memory. "We are postulating they would need to have been heavy bags to carry the device inside them. They would not have been light bags," the FBI chief said.

The sight of someone carrying a heavy black bag might also be significant in terms of video footage recovered from the crime scene. Forensic experts from the FBI laboratory in Quantico, Virginia, who specialise in the analysis of digital film, have arrived in Boston and will be involved in the massive task of combing through the voluminous mobile phone videos handed in from citizens as well as footage retrieved from security cameras in the vicinity of the bombing.

Flowers were placed at the scene of the Boston Marathon bombs. Photograph: Scott Eisen/HotSpot Media

While the forensic developments sound promising, a large vacuum remains at the heart of the investigation: there remains no knowledge about the perpetrator or perpetrators and what motivated them. DesLauriers said in this regard that the hunt remained in its infancy. "At this time there are no claims of responsibility. The range of suspects and motives remains wide open."

He added that there was no idea whether those responsible were "a person or persons".

As Boston tried to get back to normal business on Tuesday, it was like a town cut in half. On the one hand, the historic town centre was looking its beautiful best, with visitors dining on the sidewalks under a cloudless sky and the brownstone streets adjacent to the bomb site lined with magnolia trees in bloom.

On the other hand, Swat teams armed with machine-guns were posted on most street corners and outside the hospitals, and a 12-block area around Boylston Street that formed the crime scene looked as though it had been subject to a military coup. The zone was swarming with detectives, the streets jammed with armoured vehicles, and the air resounded with the barking of sniffer dogs.

Public officials gave conflicting information about the possible use of pressure cookers. DesLauriers would not specify whether they believed a pressure cooker was used in both devices. But after the press conference ended, the US prosecutor Carmen Ortiz told reporters that a pressure cooker may have been used at both sites.

Public appeal

Massachusetts governor Deval Patrick listens as FBI special agent Richard DesLauriers briefs reporters during a news conference. Photograph: Jessica Rinaldi/Reuters

The FBI chief said he wanted to hear from any member of the public who had come across a friend, colleague or acquaintance who had expressed in recent months a desire to target the marathon; who had engaged in suspicious research on explosive devices; or who appeared to be carrying an unusually heavy dark bag in the vicinity of the blasts on Monday. DesLauriers also asked people to come forward if they had heard noises of explosions before yesterday that might have been test runs for the outrage.

Ed Davis, the Boston police commissioner, denied that the force had made any errors in the lead up to the marathon. He said the number of police officers had been increased at this year's marathon, particularly at the finish line, to match the large crowd that assembled to watch the end of the race.

"This is a soft target," he said. "Anybody can go into a church service and do this kind of thing. When you have an event like this, it can't be closed down like a military operation – by virtue of the event, it requires that we don't turn it into a police state."

While much of the public focus the day after the bombing is focused on the hunt for those who carried it out, the city is also working hard to support the victims and their families. Thomas Menino, the mayor of Boston, said he had visited wounded people in hospital.

"They are hanging in there," he said. "I saw a woman who had lost a leg – she showed a lot of courage. These families are going through very difficult times, and we have to show them all the help we can."