British Airways' parent company International Airlines Group says it has signed a non-disclosure agreement with American Airlines, allowing it to look at the company's books.
Foreign investors are prohibited from owning more than 25 per cent of a U.S. airline.
The two airlines already have a joint business agreement for flights across the Atlantic Ocean and were founding members of the OneWorld frequent flier alliance.
Earlier on Friday, US Airways and American's parent, AMR Corp., announced that the two airlines had signed similar agreements, moving them one step closer to a potential merger.
AMR Corp. entered bankruptcy protection on Nov. 29, 2011.
American Airlines CEO Tom Horton has said his airline is weighing several options, including remaining independent or merging with one of several airlines.