At least 130 people are injured and three dead after two bombs exploded near the finish line of the Boston Marathon Monday afternoon. The injuries include dismemberment, witnesses said, and local hospitals say they are treating shrapnel wounds, open fractures and limb injuries. An eight-year-old boy is one of the three known dead, multiple news outlets reported, and several of the injured are also children.

At a Monday night press conference, Gov. Deval Patrick urged Bostonians to be vigilant on their morning commute Tuesday, and to report any suspicious packages to the police. The FBI has officially taken over the investigation, and is treating it as a potential terrorist attack.

Boston Police Commissioner Ed Davis stressed that the police had no suspect in custody yet. "I'm not prepared to say we are at ease at this time," Davis said, when asked if the area was safe.

"This cowardly act will not be taken in stride," Davis said. "We will turn over every rock to find out who is responsible.''

Davis said Boston police were not aware of any specific threat to the marathon before it began. Police said they had no one in custody and no suspects, but the Boston Globe reported that a "person of interest" who was injured in the blast was being questioned at Brigham and Woman's Hospital Monday night.

[Related: Full coverage of the Boston marathon explosions]

Two large explosions, just 50 yards apart, went off at 2:50 p.m. ET, more than four hours into the race. One of the explosions happened near the entrance of the Fairmont Copley Hotel in Copley Square. The blast scattered hundreds of onlookers and runners, and left a bloody scene of injured spectators, including children. Local news reporter Jackie Bruno wrote that she saw some people with their limbs blown off. The Boston Police Department said it is looking for video footage taken from the finish line as part of its investigation. Video footage shows first responders and bystanders rushing to the scene of the blast to help the wounded.

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Boston Medical Center took in 20 patients, including two children, most of whom are being treated for "lower leg injuries," a spokeswoman said. A spokeswoman for Tufts Medical Center said the hospital is treating nine patients for conditions such as shrapnel wounds, ruptured ear drums, and "serious orthopedic and neuromuscular trauma to the lower legs." At least one patient was as young as three years old.

President Barack Obama warned Americans in a brief statement Monday evening not to jump to conclusions before authorities find out who committed the crime. "We will find out who did this," Obama said in an appearance in the White House briefing room. "Any responsible individuals, any responsible groups will feel the full weight of justice." A White House official said the incident is being treated as an act of terror.

NBC News, citing anonymous law enforcement sources, reported Monday that a "small homemade bomb" is believed to be responsible for the explosion. The FAA created a no-fly zone around the area. Family and friends of marathon runners or spectators can call 617-635-4500 for information on their loved ones.

This video from the Boston Globe shows the moment the bomb went off, and the paper has also pulled together dramatic photos from the aftermath. According to marathon officials, several thousand runners had not finished the race when the explosions detonated.

Police have evacuated the area on Boylston Street to continue sweeping for more devices. Runners who had not yet finished the race were stopped at mile 25 and directed to Boston Common. The Boston Police Department called in all off duty officers in the city. This New York Times map shows where on the route the explosions took place.

[Related: Photos from the scene of the Boston Marathon explosions]

Patrick called it a "horrific day in Boston" in a statement.

The New York Police Department is stepping up security around the city in response to the explosion. At the White House, yellow police tape was used to block off Pennsylvania Avenue from pedestrians in front of the White House's north gates and secret service were positioned along the perimeter. Credentialed pass holders continued to be permitted entry and exit from both the White House and the Executive Office Building.

--The Associated Press contributed to this report.