The Boston investment giant­ Fidelity Management and Research faces a lawsuit alleging it mishandled interest earned on clients’ retirement accounts, using some of the money to pay unauthorized fees to itself.

A complaint seeking class-action status was filed Tuesday in US District Court in Boston but offered little insight into the scope of the alleged abuse. Gregory Y. Porter, an attorney for the plaintiffs, estimated Fidelity could be liable for as much as $2 billion in damages, based on a similar case in Missouri that Fidelity lost last year.