Simplify the following text for a Grade 8 native English-speaking person.
Original: Timothy McVeigh was executed by the federal government June 11 , 2001 , while his partner Terry Nichols is currently serving life in prison without the possibility of parole .
Simplified: This was the worst terrorism attack in the United States before the World Trade Center attacks on September 11 , 2001 . McVeigh and his partner , Terry Nichols , were both convicted of the bombing .