From Longman Dictionary of Contemporary Englishat libertyat libertyformalFREE/NOT IN PRISON if a prisoner or an animal is at liberty, they are no longer in prison or enclosed in a small place SYN free → liberty
Examples from the Corpus
at liberty• Tonight only one of the escaped prisoners remains at liberty; the other seven are now back in Central Prison.• Walker was at liberty for many years before the police apprehended him. 