Simplify the following text for a Grade 10 native English-speaking person.
Original: The ten-month investigation of the Warren Commission of 1963 â `` 1964 concluded that the President was assassinated by Lee Harvey Oswald acting alone and that Jack Ruby acted alone when he killed Oswald before he could stand trial .
Simplified: A ten-month investigation into what happened was done by the Warren Commission in 1963 â `` 1964 . The commission concluded that the President was assassinated by Lee Harvey Oswald .