Part of the investigation centres on claims that guns used that night may be linked to as many as 90 other murders and attempted murders.

BBC: IRA 'responsible for Kingsmills'

But, as the Supreme Court told President Truman when he attempted to use an executive order to place all steel factories under control of the federal government, executive orders may not be used to make laws, only to execute them.

FORBES: President Obama's Executive Power End Run Around The Constitution