Simplify the following text for a Grade 11 native English-speaking person.
Original: After imprisonment in Dartmoor , Maidstone and Lewes prisons in England , de Valera and other prisoners were released under an amnesty in June 1917 .
Simplified: After imprisonment in Dartmoor , Maidstone and Lewes prisons , de Valera and his comrades were released under an amnesty in June 1917 .