UPDATE 12:12 PM: ABC’s World News With Diane Sawyer is expanding to one hour again tonight as part of the network’s continuing coverage of the Boston bombings. ABC News also says it will air a special edition of Nightline with Terry Moran anchoring From Boston. That’s the only change from the networks announced so far. Both NBC’s Brian Williams and CBS’ Scott Pelley are anchoring their networks news from Boston tonight. And Fox News’ Shep Smith also will again be anchoring the network’s coverage from Boston at 3 PM and 7 PM ET. CNN’s Anderson Cooper will continue to report from Boston with his AC 360 live at 8 PM and 10 PM ET. Cooper will be joined by Chris Cuomo, Jake Tapper and Dr. Sanjay Gupta among others in CNN’s coverage. CBS said it is going back to its regular programming tonight. Two bombs exploded yesterday at the finish line of the Boston Marathon, killing 3 people and injuring more than 170. This morning President Obama called the attack “heinous and cowardly” and said the FBI is investigating it as an act of terrorism, though they still don’t know who’s behind the bombings.

Tonight’s Boston Celtics-Indiana Pacers game in Boston was cancelled yesterday by the NBA after the blasts, and other upcoming major sporting events are taking extra steps in security planning for their events including the Kentucky Derby and Indianapolis 500.

PREVIOUS MONDAY PM: NBC News will air a one-hour primetime special at 10 PM ET tonight on the Boston Marathon explosions. So far, NBC is the only network to interrupt its primetime schedule for special coverage. That means that the new episode of the network’s Revolution scheduled for tonight will not air this week. The J.J. Abrams-produced post-apocalyptic series recently returned from a long winter break to unsteady ratings. NBC is also extending its network newscast to one hour. CBS says Scott Pelley will be anchoring an extended one-hour-and-a-half-long version of the CBS Evening News tonight from New York from 6:30 PM to 8 PM ET.

President Barack  Obama earlier spoke in a brief nationally televised address from the White House press room. Obama’s statement cut into the networks’ already pre-empted coverage of the pair of explosions earlier today at the finish line of the Boston Marathon that left at least two people dead and dozens injured. “We do not know who did this. Or why. Please make no mistake. We will get to the bottom of this. We will find out who did this and why they did this. Any responsible groups will feel the full weight of justice”, Obama said.

In other coverage, ABC News host Terry Moran will anchor Nightline from Boston tonight. ABC’s World News With Diane Sawyer will be extended to a one-hour broadcast. Also Good Morning America’s George Stephanopoulos and Josh Elliott will be reporting from Boston tomorrow morning. CNN’s Anderson Cooper will be hosting AC 360 live from Boston at 8 PM and 10 PM ET. Fox News announced that anchor Shepard Smith will be live at 5 PM ET with more Boston explosion coverage in place of the regularly scheduled Five and will be anchoring from Boston for one hour starting at midnight ET.

FNC also said that its entire primetime of Bill O’Reilly, Sean Hannity and Greta Van Susteren will all be live tonight. Van Susteren will be live from Boston for a 10 PM to midnight show. And the network’s Bill Hemmer will be anchoring from Boston tomorrow morning. Fox Business Network said that anchor Neil Cavuto will be anchoring a special live report on the network at 8 PM ET covering the explosions and potential market implications. Boston police earlier said that a third explosion occurred today at the John F. Kennedy library in the city’s Dorchester area, but have since pulled back on that, saying it may have been a fire. ABC, CBS and NBC broke in to afternoon programming to air special reports of two blasts that occurred at the marathon’s finish line. CNN, Fox News, MSNBC and CNBC also are airing special continuous coverage. Of the cable news networks, Fox News was first. The News Corp-owned network broke in at 3:06:18 PM ET with CNN close behind at 3:06:53 PM ET. MSNBC started broadcasting news of the explosions at 3:08:45 PM ET.  Fox News and CNN are broadcasting live video from Boston local stations. MSNBC has video from New England Cable News, which is owned and operated by NBC Universal.