What were the Afghan “Zero Units” that the National Guard shooting suspect reportedly worked for?

The units were known in Afghanistan for their secrecy and alleged brutality, and members were implicated in numerous extrajudicial killings of civilians, particularly during night raids.

They were considered by the U.S. and its international partners to be among the most trusted domestic forces in Afghanistan.

Source: https://www.cbsnews.com/news/national-guard-shooting-suspect-afghanistan-zero-units/