WASHINGTON (AP) — Homeland Security Secretary Janet Napolitano says there is no evidence that the bombings at the Boston Marathon are part of a wider plot.

Napolitano says DHS will maintain what she called "enhanced security measures at transportation hubs" as a precaution.

Two people briefed on the investigation tell The Associated Press that a pair of bombs packed into pressure cookers and concealed in duffel bags blew up within seconds of each other.

They spoke on condition of anonymity because they were not allowed to discuss the details publicly.

Three people were killed and more than 170 people were wounded in the blasts Monday near the finish line of the Boston Marathon.