freddie gray , 25 , died sunday a week after his arrest in baltimore .
four officers arrested him for a violation now revealed to be switchblade .
he was dragged during the arrest and witness said his legs ` looked broke '
gray was loaded into a transport van and put in restraints on way to station .
gray lapsed into coma and underwent extensive surgery at trauma center .
baltimore mayor stephanie rawlings-blake promised thorough investigation .
` whatever happened happened in the back of the van , ' she said .