JACKSON, Miss. — Twelve people who received portions of a $400 million settlement with the manufacturer of the diet drug fen-phen in 1999 have been arrested and charged with fraud for allegedly lying about taking the drug. “These people were seeking money for a drug they never took. They conspired with others who were more knowledgeable about the system. They were never entitled to any type of compensation,” FBI agent Bob Garrity said Tuesday. The defendants are accused of submitting fake pharmacy documents showing they used the diet drug. They face up to five years in prison and a $250,000 fine if convicted of conspiracy to commit an offense against the United States. The arrests Monday and Wednesday came after a yearlong federal investigation into a rural corner of southwestern Mississippi where juries have been known to return multimillion-dollar verdicts, attracting lawyers who were eager to capitalize. The FBI said it wanted to learn how individuals became part of these lawsuits and how juries were picked from an area where many people are kin or acquaintances. The $400 million settlement with nearly 800 people nationwide came after one of those so-called “jackpot” verdicts. A Jefferson County jury had awarded $150 million to five people who claimed fen-phen gave them heart and lung problems. Drug maker American Home Products, which has since changed its name to Wyeth, quickly settled the case, offering to cap damages at $400 million for the five plaintiffs and claimants in about 800 other cases.All those arrested for alleged fraud are from Jefferson County. They were released Tuesday on their own recognizance. More arrests were expected, the FBI said. American Home Products made Pondimin, the fenfluramine half of fen-phen, and Redux, a chemical cousin. About 6 million people took the drugs before they were pulled from the market in 1997 amid evidence they caused heart-valve damage in some patients. Wyeth Pharmaceuticals spokesman Douglas Petkus said Wednesday that the company cooperated with authorities, but he would not discuss any details.