Simplify the following text for a Grade 14 native English-speaking person.
Original: On September 25 , 1919 , together with a number of leftist social revolutionaries , the Underground Anarchists bombed the headquarters of the Moscow Committee of the Communist Party during a plenary meeting .
Simplified: On September 25 , 1919 , a little group of social revolutionaries and anarchists bombed the headquarters of the `` Moscow Committee '' of the Communist Party in protest at the growing repression .