"OIG learned, through employee questionnaires and interviews, that the Ambassador sometimes made inappropriate or insensitive comments on topics generally considered Equal Employment Opportunity (EEO)-sensitive, such as religion, sex or colour", the report said.
The OIG said that the State Department is warranted to further investigate the allegations.
According to US law, offensive or derogatory comments based on an individual's race, colour, sex, or religion can create an offensive working environment and potentially lead to a violation of Equal Employment Opportunity laws.
The OIG said in the report that it found that Ambassador Johnson's demanding and hard-driving work style had a negative effect on morale for some staff in the US embassy in the United Kingdom.