Attorney General Jim Hood says Barbour, who left office Tuesday, may have violated the state Constitution, The Jackson (Miss.) Clarion-Ledger reported.
Under the state Constitution, a legal newspaper notice must be published in the county of conviction 30 days prior to granting a pardon. Officials said the notices failed to run on time in many cases and in some cases never ran at all.
"There is a sufficient threat of irreparable injury should the subject individuals be released based upon the purported gubernatorial pardons," Hinds County Circuit Judge Tomie Green said in blocking the pardons.
Five former prisoners, including four convicted killers, were released Sunday. Green ordered them to report each day to the state Department of Corrections and to appear in her courtroom on Jan. 23.
Hood said his office is looking at each pardon to see if the requirements were met.
"For the safety of all our residents and for the sanctity of our Constitution, we had to request the injunction. Each of these individuals will have a day in court to show that they have met the necessary requirements. If they have not, then we will ask the court to void those pardons."
Barbour granted 203 pardons or early releases to felons as he left office, the newspaper reported. Barbour said 90 percent of those who received clemency were no longer behind bars and "a majority of them had been out for years."