NEW YORK — Wells Fargo Securities downgraded American International Group. Inc. to its lowest rating Friday, saying the insurer has “virtually no tangible book value at the moment” and the recent volatility in its shares, including a huge rally in August, is a mystery.

Hall said AIG’s book value, or its net worth, is about $21.80 per common share. And taking out some of the “lower quality components,” such as a line of credit with the Federal Reserve and Treasury, he said the company’s “tangible book value” is presently nil — or even negative if one takes out goodwill.

The U.S. owns 80 percent of AIG. Hall calculates total government support at $192.4 billion. The government made huge investments in AIG to stop the company from failing amid fears it lacked the cash to make good on its insurance contracts. AIG insured complicated financial instruments, many of them tied to real estate, that were defaulting as the housing market crashed.