(a) We or the State agency will secure from the claimant or other sources any evidence the State agency needs to make a disability determination. When we secure the evidence, we will furnish it to the State agency for use in making the disability determination.

(b) At our request, the State agency will obtain and furnish medical or other evidence and provide assistance as may be necessary for us to carry out our responsibility for making disability determinations in those classes of cases described in the written guidelines for which the State agency does not make the determination.