WASHINGTON >> President Barack Obama urged a nervous nation to avoid jumping to conclusions about today’s explosions at the Boston Marathon, while acknowledging that officials “still do not know who did this or why.” A White House official later said the incident was being treated as terrorism.

Speaking from the White House just three hours after the explosions, Obama was cautious in his remarks, stopping short of calling the incident an act of terror.

Obama said the explosions were the act of an individual or group, but vowed to hold those responsible accountable.

“We will find out who did this. We’ll find out why they did this,” Obama said in his brief, three-minute statement. “Any responsible individuals, any responsible groups, will feel the full weight of justice.”

The White House official spoke on condition of anonymity because the investigation was ongoing.

The president said the government would increase security around the United States “as necessary” but did not say whether his administration thought the incident was part of a larger plot.

Obama did not offer specific details on deaths or injuries, saying only that multiple people had been wounded, some of them gravely.

Authorities say at least two people were killed and more than 80 injured during two explosions near the finish of the marathon. A senior U.S. intelligence official said two other explosive devices were found near the end of the 26.2-mile course.

The president was briefed on the incident today by several senior administration officials, including FBI Director Robert Mueller and Homeland Security Secretary Janet Napolitano. He also spoke with Massachusetts Gov. Deval Patrick and Boston Mayor Tom Menino and pledged to provide whatever federal support was needed in responding to the incident.

Additionally, the president spoke with Republican and Democratic leaders on Capitol Hill, saying that “on days like this, there are no Republicans or Democrats, we are Americans united in our concern for our fellow citizens.”

As the president was being briefed on the incident, the Secret Service quickly expanded its security perimeter at the White House. The agency shut down Pennsylvania Avenue and cordoned off the area with yellow police tape. Several Secret Service patrol cars also blocked off the entry points to the road.

The White House was not on lockdown and tourists and other onlookers were still able to be in the park across the street from the executive mansion.

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Associated Press writer Nedra Pickler contributed to this report.