West Midlands Police confirmed that the information was taken from the parked car in Walsall.
An urgent investigation was launched following the theft on September 1. Four men were later arrested.
Two remain on bail while the other two have been released under investigation, West Midlands Police said today.
According to a formal letter, which has been sent to those affected, the property stolen included police documentation and a note book containing names, addresses, vehicles and dates of birth for a number of people linked to criminality and gangs within the Walsall area.
Police chiefs have confirmed the information contained details of people "assessed as being involved in, or vulnerable to, gang-related activities", but say they do not believe the loss has directly put anyone at risk.
West Midlands Police said the data loss is being investigated by its professional standards department and an investigation has also been launched by the Information Commissioner’s Office.