This morning investigators looking into what caused this engine to explode over Colorado are zeroing in on a weakened piece of metal. New images released by the NTSB shows the damage to the plane including a gaping hole under the wing. Suggesting some of the debris struck the plane itself. There were also dings in. And Nixon and other places on the wing but there was no structural damage another image shows what investigators believe caused the explosion two broken fan blades and the engine. The emergency on the united flight from Denver to why he started four minutes after takeoff according to the NTSB's preliminary investigation a fan blade broke off. Tearing into an adjacent blade investigators say the first play to separate showed signs of metal fatigue and metal fatigue. You usually going to find what I won't describe his speech marks it will be a serious. Of marks. That you can do visibly see me. Often times with the naked ally once you put it on her plate and scanning electron microscope. Aren't you can actually count the number of cycles since the initiation. Of. That fatigue the broken pieces of a metal blades consider key pieces of evidence have now been recovered one of them was lost inside the plane's engine the other fell onto a neighborhood soccer field overnight the NTSB said the boys were being transported on a private flight to the engine maker Pratt and Whitney flab in Connecticut. Where investigators can get a closer look Pratt and Whitney says it is actively coordinating with regulators the NTSB says the agency will compete here Saturdays incidental. To other emergencies in the past. Which have involved that same model engine but we are in the business of determining probable cause we did not determine liability would want to find out what happened. So that we can issue recommendations to keep it from happening again. United Airlines has grounded as other Boeing 77 sevens with the same engine the FAA has order more inspections.