The stakes are being sold off by J.P. Morgan, which secured partial ownership as part of a $1.75 billion deal with Simon in 1999 to acquire 14 malls from local developer Stephen R. Karp’s New England Development Co. An official with Eastdil Secured, which is marketing the properties, declined to comment.

The Atrium Mall, once considered a prime retail center, has seen increasing competition in recent years with the expansion of shopping centers in Dedham and Natick. Moreover, one of the Atrium’s anchors, Borders, the struggling book store chain, is not expected to renew its lease when it expires, according to the people briefed on the matter. The entire portfolio on the market could fetch more than $500 million, according to estimates by the people briefed on the matter, who declined to comment because the details are not public.