Aon and GE Capital were limited partners in a financial operation now at the centre of the Enron scandal.
The world's biggest broker and GE Financial Insurance Co's parent organisation were among other reputable global companies involved in an off-balance sheet partnership called LJM2 Co-Investment.
LJM2 and Enron's other partnerships are the subject of frenzied attention from international media, regulators and congressional investigators in the US.
The off-balance sheet vehicles were used to enhance Enron's earnings and credit rating. Under US accounting rules, such entities can be treated as off-balance sheet if at least 3% of the capital is owned by company outsiders.
For a while, the partnerships' accumulation of huge debts helped Enron's balance sheet look good and buoyed investor confidence. But Enron was forced by the US Securities and Exchange Commission in November to restate financial results and reveal the extent of debt previously held off its balance sheet.
Investor confidence collapsed and Enron filed for bankruptcy the following month.
According to claims and counter-claims filed in a Delaware court in January, at the time that Enron filed for bankruptcy, LJM2's partners still included names such as Citigroup, JP Morgan Chase and Deutsche Bank.
It is not known if Aon was still a partner at the time.
GE Capital remains a partner. Its current investment is worth $750,000 (£530, 000).
Confidential papers listing Aon and GE Capital among other partners in LJM2 at a meeting on 26 October 2000 were published by a US financial website.
The papers reveal that of LJM2's total capital commitments of $394m (£278m), 42% was from financial institutions and insurance companies.
The Committee on Energy and Commerce, which is undertaking one investigation, in December demanded all documents relating to LJM2 and other partnerships.
It has not been established whether Enron's use of its partnerships was fraudulent and setting up such partnerships is common practice in US business.
LJM2 was run by former Enron chief financial officer Andrew Fastow.
US lawyer Stanley Grossman, an attorney at New York law firm Pomerantz, Haudek, Block, Grossman & Gross, said outside investors in Enron's off-balance sheet partnerships could be open to legal action under US law.
He argued partners in LJM2 knew Fastow was both Enron's finance chief and LJM2's general partner, creating a conflict of interest and the potential for insider trading.
An Enron board special committee report released at the weekend found Fastow used a similar limited partnership to turn a $25,000
(£ 17,600) family contribution into $4.5m (£3.12m) in two months.
The deal violated Enron's conflict of interest rules.
Grossman said: "Fastow was wearing two hats. He told investors in some of those partnerships, so I understand, that because of his dual position it would be an attractive investment for them.
"He would be able to take advantage of information otherwise not available. In addition, they knew he was going to personally benefit.
"But they went into these partnerships knowing he was in a conflicted position, knowing he was going to benefit. They [the partners] may well be setting themselves up for an action."
Grossman represents Enron lenders who lost nearly $120m (£84.8m) to the energy trading company.
The precise nature of Aon's dealings in LJM2 are not known.
Insurance Times contacted Aon, but the company was unable to respond by the time the magazine went to press.