NEW YORK (Bloomberg) — Bermuda-based Assured Guaranty Ltd. chief executive officer Dominic Frederico said Standard & Poor's decision to strip the bond insurer of the industry's last AAA rating last month was "unwarranted" and appeared to have been based on a "truly unimaginable" scenario.

S&P "didn't provide us with certain critical assumptions and key sensitivities" it used in its analysis, Frederico said on Friday in a conference call with analysts about the company's third-quarter earnings.

He said he's calling for a regulator to oversee the ratings companies to "facilitate a more transparent and consistent business environment for all issuers and investors."