The Securities and Exchange Commission’s investigation into the collapse of Lehman Brothers Holdings Inc. is zeroing in on an accounting maneuver used to give the appearance that the company had reduced its debt levels, according to people familiar with the situation. Agency officials also are examining whether former Lehman executives failed to adequately mark down the value of the huge real-estate portfolio acquired in the securities firm’s takeover of apartment developer Archstone-Smith Trust or to disclose the resulting losses to investors, these people said. The narrowing probe could move the SEC closer to bringing civil charges related to Lehman’s collapse in September 2008, though a decision doesn’t appear imminent.