Boston Marathon explosions

Boston Marathon explosions - The deadly bombing at the Boston Marathon that killed at least three and injured at least 144 is believed to be an act of terrorism, senior White House officials told Fox News.

Two explosions tore through the finish line of the world-famous race just before 3 p.m., going off simultaneously as throngs of onlookers watched runners complete the 26.2-mile trek. The timing of the blasts immediately sparked suspicions of a deliberate act.

“When multiple devices go off, that’s an act of terrorism,” a senior administration official told Fox News, just moments after President Obama delivered a statement to the nation and did not use the word “terror.”

The official stressed that it was unclear whether a foreign entity like Al Qaeda was behind it or whether it was home-grown terrorism, prompting  the president’s caution in characterizing the bombing.

A first responder source confirms to Fox News that five total explosive devices were found in the Boston area, including the two that exploded. Authorities spent the next several hours sweeping the area for additional devices.

A source involved in the investigation tells FoxNews.com police are searching an apartment in the nearby Boston suburb of Revere as part of the investigation into the explosions. The apartment is on the fifth floor of the building.

Investigators were seen leaving the Revere house early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag, according to the Associated Press.

Authorities are guarding a person of interest at a local hospital, according to a New York Post report confirmed by Fox News.

The person, who sources said was 20 years old, had severe burns, but authorities had not determined whether the person was a victim or a perpetrator. Sources say the person of interest is reportedly a Saudi national in the United States on a student visa.

Read more: foxnews