Same word as circumstantiate:

affirm, approve, attest, authenticate, back, bear out, bless, buy, certify, check, check out, circumstantiate, corroborate, debunk, double-check, endorse, establish, explain, give green light, give high sign, give stamp of approval, give the go-ahead, give the nod, justify, lap up, make good, make sure, okay, rubber-stamp, sanction, settle, sign, sign off on, size up, subscribe, substantiate, support, thumbs up, underpin, uphold, verify, vouch, warrant, witness