NSA and FBI both approached previous bitlocker devs to insert backdoors in the early days. It's no secret Microsoft cooperates with federal government branches to ensure the government keeps using their products. Further because bitlocker is closed source, there hasn't been any outside research done on the code.

https://boingboing.net/2013/09/11/how-the-feds-asked-microso...

Some good comments here:

https://old.reddit.com/r/sysadmin/comments/26vm25/why_is_the...

There's more resources on google and I remember attending a talk at either blackhat or defcon on why you shouldn't be using bitlocker.