PHILADELPHIA (AP) —  The U.S. Securities and Exchange Commission has joined a federal probe of Pennsylvania’s largest pension fund and is seeking records to determine whether improper “compensation and gifts” may have been offered staff. The commission is also seeking records about what officials of the $64 billion Public School Employees’ Retirement System have called a mistake in calculating the fund’s long-term investment performance. They are also seeking documents on the the risk-sharing calculations and information on the fund’s purchases of parcels of land in downtown Harrisburg.