The Accident Group, which ceased trading last week, is at the centre of a fraudulent claims investigation.
Bus company Arriva has hired private detectives to scrutinise more than 400 claims submitted by the Manchester-based injury claims firm.
The Accident Group's parent company, the Amulet Group, was placed into administration last Friday.
About 2,500 staff members lost their jobs, many learning their fate by text message.
A spokesman for Arriva said: "We have engaged a private investigative agency to check the validity of claims being made by representatives of The Accident Group.
"We can confirm that during a sixth-month period, over 400 claims were submitted by representatives of The Accident Group.
"Subsequently a number were withdrawn, a number of people were uncontactable and in a number of cases fraud was suspected."
Accountancy firm PricewaterhouseCoopers has been appointed administrators.
No one was available to comment at The Accident Group offices on Monday.