Federal prosecutors will not bring criminal charges against current and former American International Group Inc. executives for their role surrounding financial contracts that nearly brought down the insurer about two years ago, according to people familiar with the matter.

The decision brings to a close a criminal investigation that, while mostly under wraps, was widely followed. The September 2008 bailout of AIG was one of the biggest and most shocking of the financial crisis, as trading by a noninsurance unit brought down one of the most iconic financial companies world-wide.

The probe focused on Joseph Cassano, who headed a London-based unit of AIG called Financial Products, people familiar with the matter have said. Other executives at the unit, Andrew Forster and Tom Athan, also were targets of the investigation, these people said.

"The system worked," said lawyers F. Joseph Warin and Jim Walden of Gibson, Dunn & Crutcher LLP, who represent Mr. Cassano, in a statement on Friday. "The large group of federal agents and prosecutors was diligent and professional throughout the investigation, and our client is grateful that they did their jobs by following the facts to the end."

David M. Brodsky and Richard Owens of Latham & Watkins LLP, who represented Mr. Forster, said Friday that "the facts were stronger than the emotions surrounding AIG's problems," and that they knew they could convince prosecutors "that our client at all times acted in good faith."