BREAKING: People trapped after Pennsylvania steel plant explosion

We have some breaking news. A rescue mission now underway in Pennsylvania to try to free people trapped underneath the rubble after a steel plant explosion near Pittsburgh. You can see these pictures from the scene.We're getting our information from an Alageney County spokesperson who also tells NBC News that the health department is on site testing the air quality. Now, it's early on. We don't know how many people are trapped.Also, the Associated Press reports there are multiple injuries, but exactly how m ...