Philadelphia police said Tuesday it is unclear whether the incident in Northern Liberties was connected to May Day. May 1 is traditionally a rallying day for anti-capitalist and anarchist groups, dating to the 19th century and commemorating the Haymarket riot of 1886. That day in Chicago, someone lobbed a bomb at police during a rally for an eight-hour workday, and gunfire broke out. Several police officers and civilians were killed and dozens were wounded. The eight labor activists convicted of conspiracy in the aftermath — four of whom were executed — were pardoned seven years later by Illinois Gov. John Atgeld, who wrote there was no evidence they had been involved in the bombing.