I was flying the first of 4 USAF aircraft on a simulated bombing run on a bridge span in Dallas, Texas. The other 3 aircraft were behind me, about 10 miles apart. We were all flying about 20,000 feet.

The radar weapons officer called the cockpit, very excited, and asked if we had seen "something" pass under the aircraft from front to rear?   We hadn't.   He calculated its speed as "at least two thousand" miles per hour.  
We were about to dismiss this as a radar glitch when the aircraft behind us called us to ask if we had something pass under our airplane at "several thousand miles per hour".  
Then the third and forth airplane called to say they had the same experience.  
Several weapons officers were on each aircraft and all were watching their radar screens and all aboard each aircraft saw the object.  
This was in 1957 when there weren't any aircraft capable of the speeds we saw that night.