Distance running greats Haile Gebrselassie and Paula Radcliffe were among those stunned by the twin blasts that left at least two dead and dozens injured at the Boston Marathon on Monday.

“Horrified to hear news of bomb explosion near Boston marathon finish,” Britain’s Radcliffe said on Twitter before the extent of the damage was known.

“Situation looks awful, thoughts with everyone,” the fastest women’s marathoner in history added later. “There are some very sick people out there, who would do something like this?”

Ethiopia’s Gebrselassie deplored what marathon organizers said was a bomb attack, although law enforcement officials did not immediately confirm the cause of the explosions.

“Running brings people together, but what just happened in Boston is terrible,” he said on Twitter. “My thoughts are with everybody in Boston.”

Race organizers said on Facebook that “two bombs” exploded near the finish line, without providing a source for the information.