Boston marathon attack
Boston marathon attack Reuters

Homeland Security Secretary Janet Napolitano said on Tuesday that there is no evidence that the bombings at the Boston Marathon on Monday are part of a wider plot.

Napolitano said that the Department of Homeland Security will maintain what she called “enhanced security measures at transportation hubs” as a precaution.

She urged the American public to remain vigilant and to immediately report any signs of suspicious activity to local law enforcement officials.

Three people were killed in Monday’s explosions, which U.S. President Barack Obama condemned on Tuesday as "an act of terror." He refrained from referring to the explosions as an act of terror in his first news conference on the subject, hours after the attack.

Obama said there was still no indication as to whether domestic or foreign terrorists carried out the attack and no claim of responsibility was made.

Investigators quoted by several U.S. media outlets said the bombs were hidden in pressure cookers put in backpacks. The attackers packed the bombs with metal pellets and nails to cause maximum suffering.

An eight-year-old boy who was killed in the bombing attack was identified as Martin Richard.

He was waiting, together with his mother and sister, to give his father, who was participating in the race, a hug as he crossed the finish line when the bombs went off.

In addition to killing Martin, the bomb took his sister’s leg and injured his mother.

Massachusetts resident Krystle Campbell, 29, was also named as one of the dead.

Security was boosted across the United States on Tuesday after the double blast, and Obama ordered federal agencies to assist police and local officials prevent any further attacks by increasing vigilance.

"We're continuing to monitor and respond to the situation as it unfolds," Obama said.