The 26-year-old man, arrested under the Terrorism Act, has been taken to the Serious Crime Suite at Musgrave Police Station in Belfast where he will be interviewed.
Five other men (aged 20, 41, 54 and two 21-year-olds) who were arrested this morning, remain in police custody at this time.
Ms McKee was shot dead while observing riots in Creggan’s Fanad Drive on Thursday, April 18, 2019.
A 18-year-old New IRA gunman fired the fatal shot as she stood beside a police landrover.
He later fled to Donegal and has since become a father.
A number of Derry men have already been charged with her murder.