The panel was convened recently and will sit three days a week for six months. It is likely to hear several matters — not just the Trump case ­— during the duration of its term, which is longer than a traditional New York state grand-jury assignment, these people said. Like others, they spoke on the condition of anonymity to discuss an ongoing investigation. Generally, special grand juries such as this one are convened to participate in long-term matters rather than to hear evidence of crimes charged routinely. The move indicates that District Attorney Cyrus R. Vance Jr.’s investigation of the former president and his business has reached an advanced stage after more than two years. It suggests, too, that Vance believes he has found evidence of a crime — if not by Trump then by someone potentially close to him or by his company. Vance’s investigation is expansive, according to people familiar the probe and public disclosures made during related litigation. His investigators are scrutinizing Trump’s business practices before he was president, including whether the value of specific properties in the Trump Organization’s real estate portfolio were manipulated in a way that defrauded banks and insurance companies, and if any tax benefits were obtained illegally through unscrupulous asset valuation.