An FBI agent pleaded guilty today to manslaughter in the strangulation death of a pregnant woman with whom he purportedly was having an affair. Pike Circuit Judge Bayard Collier sentenced the agent, Mark Putnam, to 16 years in prison after Putnam accepted a plea bargain and pleaded guilty to killing Susan Daniels Smith, 28, of Freeburn, Ky., on June 8, 1989. Sources have said that Ms. Smith met Putnam when she was serving as an FBI informant. The Pike County grand jury indicted Putnam earlier today. The indictment said Putnam killed the woman ``while under extreme emotional duress.'' The FBI has said it has no record of any agent ever being convicted of killing someone in a similar case. Dressed in a starched white shirt, blue jeans and tennis shoes, Putnam said little during the 18-minute proceeding, except to answer Collier's questions. Ms. Smith's family on Monday bitterly denounced the pending plea agreement and asked the grand jury to indict him for murder. Shelby Ward, Ms. Smith's sister, and several sources say Putnam admitted killing Ms. Smith during an argument over support payments for the woman's expected child. The sources, who spoke on condition of anonymity, and Ms. Smith's family say the agent described the killing and location of the woman's body in exchange for a plea bargain. The remains were found only last week. Mrs. Ward and her brother, Billy Joe Daniels, said Commonwealth's Attorney John Paul Runyon did not consult the family about the plea bargain. ``Because he was a big FBI, famous and everything, they don't want him to get justice,'' Mrs. Ward said after meeting with the grand jury. ``I'd still like to have a murder trial. ... I don't care what that clique there says. To me it was a brutal murder. Brutal, brutal.'' Runyon declined to respond, saying only: ``I can't control what she says.'' Ms. Smith's family has retained Pikeville lawyer Larry Webster to file a lawsuit against the agent and possibly the FBI. Webster said he intends to file the suit today, seeking ``millions'' in damages and lost earnings. ``If she was working for them at the time and she lost her life, she could be entitled to get paid for that,'' Webster said.