Before 3 p.m. ET on Monday, April 15, two explosions at the Boston Marathon killed three people and injured 176 others, and turned the race into what ABC News said resembled "a war zone."

The explosions occurred almost simultaneously near the race's finish line on Boylston Street, which was crowded with runners and spectators, according to ABC. Thousands of runners were still completing the race at the time of the first explosion.

Massachusetts Gov. Deval Patrick and some city agencies took to social media to inform citizens of the incident. The Boston Police Department, for instance, said via Twitter that it's looking for video footage from the race's finish line, and is sharing other relevant information.

In addition to sharing information, such as the tweets embedded below, Patrick asked those who saw anything suspicious to call 1-800-494-TIPS or the Mayor’s Hotline: 617-635-4500.

Though #bostonmarathon began as a way to communicate about the race, it is now a hashtag that links to updates about the attack.

In addition, Google set up a person finder specifically for the Boston Marathon explosions, where anyone who has information related to a found person can enter that information, or anyone looking for a missing person can conduct a search.

At 6:10 p.m. ET, President Obama addressed the nation. "We do not know who did this, or why," he said. "But make no mistake. We will get to the bottom of this."

Main photo: Medical responders run an injured man past the finish line the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/Charles Krupa)