Two explosions set off near the finish line of the Boston Marathon killed or injured multiple people Monday, the Boston Herald reports.

“I saw two explosions. The first one was beyond the finish line. I heard a loud bang and I saw smoke rising,” said Herald reporter Chris Cassidy, who ran in the marathon.

Chris McIntosh, publisher of the Boston Business Journal, said he heard two separate explosions about five seconds apart just before 3 p.m. eastern time.

Fox News quoted a law enforcement source as saying at least three people are dead in the explosion.

Many of the runners in the marathon were still on the course when the explosion happened. The finish line is now being evacuated, along with the nearby Lenox Hotel.

It's not yet known whether the explosions were a terrorist attack.

Video of the aftermath of one of the explosions shows bloodied sidewalks and evacuated seats along the race route.