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No, this is not legitimate.
The Office of Personnel Management manages civil servants of the US government. They do not investigate criminal activity. Also if you were a suspect in such an event, you would get a visit from the FBI.
This is a letter from a scammer, that is meant to scare you into giving your personal information online. In the "registration," they would get your full name, DOB, SSN, address, possibly financial information - more than enough to steal your identity.
THIS IS A SCAM. IGNORE.
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