Massachusetts announced it has reached a settlement with LPL Financial LLC that will require the firm to reimburse investors who were improperly sold securities by the firm over a 12-year period.

As a result of the settlement, LPL has agreed to buy back illegally sold securities from investors not only in Massachusetts, but nationwide, at face value with 3 percent interest.

The customer reparations are in addition to a $26 million administrative fine LPL agreed to pay as part of a multi-state settlement last month that found the firm failed to register and supervise the sale of securities to brokerage customers beginning in October 1, 2006.

The full dollar amount of reimbursements LPL must pay has not been calculated yet, said Debra O’Malley, a spokesperson for the Massachusetts Office of the Secretary of the Commonwealth.

“This agreement will give Massachusetts investors who were misled when they were offered these unregistered securities the chance to get their money back, with interest, and re-invest it in a way that works best for them,” said state Secretary of the Commonwealth William F. Galvin, who led the investigation along with Alabama Securities Commissioner Joseph Borg.

“Many of these investors purchased these unregistered securities through their local agent, believing that they were compliant with the law. My office is committed to ensuring that brokers and financial institutions are not taking advantage of Massachusetts investors and their hard-earned savings and I am happy to have led this investigation along with Alabama to assist investors nationwide,” Galvin continued.

The settlement with LPL was reached after a North American Securities Administrators Association task force led by Massachusetts and Alabama determined LPL had been negligent in its duty to supervise its agents and employees, which led to the sale of unregistered securities to its customers over the past 12 years.

Customers who were sold unregistered, non-exempt securities since October 2006 will be offered the full amount paid, plus 3 percent interest. Interest will be calculated from the trade date of the purchase to the earlier of May 1 or the date on which the customer sold the security, if applicable, according to the settlement.

LPL has also agreed to a full review to assess its compliance with all state securities requirements, Galvin’s office said.

LPL did not immediately respond to a request for comment. Under the settlement, the firm neither admitted to nor denied the allegations.