The environmental activist Daniel McGowan was released from prison Friday afternoon after federal authorities were notified they had arrested him under a regulation declared unconstitutional. McGowan had been taken into custody Thursday just months after his release to a halfway house following over five years in prison for his role in two acts of arson as a member of the Earth Liberation Front. In his case, the judge ruled he had committed an act of terrorism, even though no one was hurt in either of the actions. McGowan was told he had violated a Bureau of Prisons rule for publishing an article decrying his treatment for The Huffington Post. McGowan’s attorneys say they won his release after pointing out that the regulation in question had been declared illegal in 2007 and eliminated in 2010.