Police arrested 22-year-old Pamela A. Smart early Wednesday afternoon, charging her with planning the murder of her husband so she could have an affair with another man.

Mrs. Smart was arrested at 1:45 p.m. Wednesday at the School Administrative Unit 21 offices on Hampton, where she works as the director of the media center, and was arraigned later that day in Derry District Court.

Police and the attorney general's office are charging her with being an accomplice to the May 1 murder of her husband, Gregory Smart, 24, of Derry. Smart's body was found by his wife in the Misty Morning townhouse in Derry with a single gunshot wound to his head.

Assistant Attorney General Cynthia White said a probable-cause hearing is scheduled for Aug. 20, but a grand jury could hand down an indictment against Smart when it reconvenes next week, making the hearing unnecessary.

The case would then go to Rockingham County Superior Court. Smart is being held without bail at the state women's prison in Goffstown. Her Chichester attorney, Paul Twomey, said yesterday he and partner Mark Sisti would be seeking a bail hearing and would be entering a plea of innocent on her behalf.

The accomplice-to-murder charge carries a mandatory penalty of life in prison without parole.

In June, police arrested three Seabrook teen-agers in connection with the murder. The attorney general's office is seeking to have them tried as adults.

In the affidavit filed in Derry District Court by Derry Police Detective Daniel Pelletier, Mrs. Smart is said to have helped plan out the murder of her husband. The affidavit was released by the court yesterday, with a substantial part of it blackened out to protect the identities of the juveniles involved.

Gregory Smart was a life insurance salesman. According to the affidavit, Mrs. Smart stood to gain from $140,000 from two Metropolitan Life Insurance policies that were payable upon the death of her husband.

The affidavit said Mrs. Smart wanted to have an affair. Divorce was out of the question because Gregory Smart "would take all the furniture, the dog and leave her with no place to live. In addition, she would not be able to date because he would follow her around," according to the affidavit.

A statement released by Attorney General John Arnold and Derry Police Chief Edward Garone accuses Pamela Smart of aiding 16-year-old "William F" in the killing of her husband. "William F purposely caused the death of Gregory Smart by shooting him in the head on or about May 1, 1990," the statement said.

The names of the three juveniles arraigned at Hampton District Court in June have not been released by officials because of privacy laws regarding minors.

Soon after the youths arrest, Mrs. Smart said she knew the juveniles and was "devastated" by the arrests.

According to Pelletier's affidavit, Mrs. Smart helped plan out the murder of her husband, telling the killer to make the murder look like Gregory Smart walked in on a burglary. During the murder investigation, police found items missing from the couple's home.

Pelletier wrote that Mrs. Smart told the killer to dress in black clothes, pull his hair back, and wear gloves, so as not to leave fingerprints during the murder. Mrs. Smart later complained to a second party because a glove and knife has been found by police near the scene of the murder.

The knife and glove were found by Derry Police in a field near the couple's townhouse, the affidavit said. The knife appeared to be part of a set found in the Smarts' kitchen, according to Pelletier's statement.

Mrs. Smart also planned to kill her husband once before, the affidavit said, but the attempt failed: Mrs. Smart had lent out her car so an unnamed party could kill her husband, Pelletier said. The attempt failed when the driver lost his way, got delayed and finally arrived to find that Gregory Smart was already at home.

Mrs. Smart sounded angry and disappointed when talking about the unsuccessful attempt to kill her husband, the affidavit said.

The affidavit said Mrs. Smart was heard on the telephone at her SAU 21 office, arguing with her husband about divorce and the disposition of property, within 21/2 weeks of the murder.

Police have recovered a .38-caliber gun used in the killing of Gregory Smart. An autopsy by state medical examiner Dr. Roger Fossum showed that Smart died from a single .38-caliber revolver.