'Act of terror'

US President Barack Obama said in a press conference on Tuesday that the Boston Marathon bombings, which killed three and injured 176, are being treated by the Federal Bureau of Investigation (FBI) as an act of terrorism.

"This was a heinous and cowardly act," Obama said in Washington. "Any time bombs are used to target innocent civilians, it is an act of terror."

He added that while the attacks are being treated as terrorism, there was no indication about who carried out the attacks or why. He said it was unclear if a foreign or domestic group was at work, or if it was the work of a "malevolent individual."

Obama praised the first responders who helped treat the victims in the initial moments after the explosions and the medical staff who continue to treat the wounded.

"If you want to know who we are, who America is, how we respond to evil - that's it," he said. "Selflessly, compassionately, unafraid."

Calls for photos

A few hours before Obama's remarks, a press conference was held in Boston featuring heads of the agencies taking part in the FBI-led investigation. Multiple reminders were issued that the public is encouraged to submit pictures of video of the blasts and the race to give authorities a better idea of what happened.

"This is probably one of the most photographed areas in the country yesterday," said Boston Police Commissioner Ed Davis.

Officials were also able to reassure the public that there are currently no threats to public safety, and that the two bombs that exploded at the finish line were the only two involved in Monday's attacks.

Massachusetts Governor Deval Patrick said that there were "no unexploded explosive devices found." Other suspicious packages that had been checked out turned out to be harmless.

No word on suspects

FBI Special Agent Rick DesLauriers added that "there's no known imminent physical threat at any location where we might be conducting investigative activity right now."

DesLauriers declined to comment on questions about any suspect the FBI was holding in custody.

Monday's blasts occurred about 10 seconds apart as marathon participants completed the final stretch of the race. At the finish line, spectators lined the streets to watch the racers come in. One of the three people killed was an eight-year-old boy, who was at the race cheering on his father. The boy's mother and sister were also seriously hurt.

Doctors from local hospitals also gave press conferences on Tuesday and said that many of the injuries were a result of shrapnel that had likely been embedded in the explosive devices. Local health officials said that a high number of victims had undergone or would require amputation surgery.

Increased police presence

According to reports in the newspapers "The Boston Globe" and the "Boston Herald," the FBI, Homeland Security and local officials searched an apartment in Revere, Massachusetts, which lies about 5 miles (8 kilometers) northeast of Boston's downtown. Officials would not confirm if the search was related to the explosions.

DesLauriers pledged that the FBI would put everything it had behind the investigation.

"It will be a worldwide investigation," he said. "We'll go to the ends of the earth to identify the subject or subjects who are responsible for this despicable crime and we will do everything we can to bring them to justice."

Streets near where the blasts took place remain closed on Tuesday, and police reminded the public that there would be an increased police presence in the coming days as the investigation is carried out.

The two explosions occurred 50 to 100 yards (45-91 meters) apart shortly before 3 p.m. local time (1900 UTC) near the north side of Bolyston Street.

The annual Boston marathon attracts over 20,000 runners and about 500,000 spectators annually. A local state holiday commemorating the first battles of the American Revolutionary War, known as Patriots' Day, coincides with the sporting event.

mz/msh (AP, AFP, Reuters, dpa)