20 CFR §10.121

If the claimant submits factual evidence, medical evidence, or both, but OWCP determines that this evidence is not sufficient to meet the burden of proof, OWCP will inform the claimant of the additional evidence needed. The claimant will be allowed at least 60 days to submit the evidence required. OWCP is not required to notify the claimant a second time if the evidence submitted in response to OWCP's first request for additional evidence is not sufficient to meet the burden of proof.