Why was bertillonage only used if a criminal was in police custody?

Because It is hard for the police to carry out the tools necessary for the measurement out side the police station

Explanation:

Bertillonage is the record keeping process that conducted by the police to record several important data about the criminals ( including their size, arm  length span, scars on their body , distinguishing features, etc)

The purpose of this record is to alert them in case the criminals that they let go repeat the same crimes again in the future. They need a lot of tools to properly measure this data, and they cannot do it unless they have them in their custody.

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