A former district attorney in south Louisiana repeatedly abused his power to prey on vulnerable women, offering them leniency from his office in exchange for sex, authorities said Wednesday after the veteran prosecutor pleaded guilty to obstructing a federal investigation of the sex abuse allegations.

Investigators accused former St. Charles Parish District Attorney Harry Morel of soliciting sex from at least 20 women during his 33-year tenure in office. In return, he offered them help with their cases or relatives' cases, they said.

"Harry Morel could make things go away, but he wanted sexual acts in exchange," U.S. Attorney Kenneth Polite said. "We suspect that this pattern of conduct has been ongoing for many decades. In fact, we will never know the full extent of it."

One of Morel's alleged victims, 27-year-old Danelle Keim, became a key witness against him. Keim died of a drug overdose in 2013, but investigators said her cooperation with an FBI investigation was instrumental in securing a guilty plea from the 73-year-old, who faces a maximum sentence of three years in prison.

A court filing Wednesday says Morel engaged in "inappropriate behavior" with Keim at her home after her arrest in St. Charles Parish on a drunken driving charge in March 2010. On Thursday, St. Charles Parish Sherif's office released the tape of Keim's 911 call after Morel left her apartment. She accused him of sexually assaulting her.

"He grabbed me, he kissed me and he touched me in my private areas," she told a dispatcher in a trembling voice. "He wanted me to take off my clothes. He wanted me to take my pants off so he can please me."

Keim later told investigators that Morel left after she pulled away from his kiss, according to a taped interview released Thursday.

"He touched me, and I didn't want to be touched. Nobody touches me if I don't want to be touched," she said.

Keim told a deputy that she worried it would be "my word against his."

Keim later recorded conversations with Morel for the FBI after he agreed to assist her with new theft and drunken driving charges, according to Wednesday's court filing. The FBI also videotaped a July 2012 meeting between Morel and Keim at her home. Morel brought two bottles of wine and again attempted to engage in "inappropriate behavior," the filing said.

Jeff Sallet, special agent in charge of the FBI office in New Orleans, said Keim "should be singled out for her tremendous bravery and resolve."

"Harry Morel is nothing short of a sexual predator," Sallet said. "His days of victimizing the most defenseless among us are over. He has been brought to justice and will now pay for his crimes."

Keim's mother, Tammy Glover, said her daughter cooperated with the FBI for more than a year.

"She went undercover with the FBI and exposed him," Glover said in a telephone interview. "If it would not have been for my daughter, who is my hero, (Morel) would have never been exposed. I am amazed at what she did and so very proud of her. She will never be forgotten for her bravery in bringing down Mr. Morel."

Wednesday's court filing says Morel solicited sex from other defendants or relatives of defendants between 2007 and 2009, offering them favorable treatment from his office. But the document doesn't provide any details of those allegations.

In the end, prosecutors did not charge Morel with any sexual crimes. He pleaded guilty to a narrowly tailored charge of obstruction of justice for harassing Keim and pressuring her to get rid of evidence in the federal grand jury investigation that targeted him.

In 2011, a boyfriend of Keim took photographs of meetings between her and Morel in a courthouse parking lot and at a satellite office for the district attorney's office. Morel instructed Keim to destroy photographic evidence of the meetings, knowing federal authorities wanted it, the court filing says.

"You shoulda got rid of it a long time ago," Morel told her during a 2012 meeting, according to the filing.

Morel declined to comment after the hearing, but his attorney, Ralph Capitelli, said officials' comments describing Morel as a sexual predator were a smear tactic to influence sentencing.

"That is both unfair and, in my judgment, impermissible," Capitelli said.

Polite said justice is a slow and "often imperfect process" and cited the statute of limitations as one reason prosecutors did not pursue the more serious sexual offenses.

"By title, he was the embodiment of justice," Polite said. "However, in the darkness of his heart, he was something else entirely -- a man who perverted his position of power to take sexual advantage of desperate women who needed help. And he did this over and over and over again."

Polite said his predecessor had decided against prosecuting Morel because of a number of legal problems, including whether they could prove explicit demands for sex in exchange for help from his office. But he said he decided the case "called for justice" once he heard the facts.

Morel's sentencing is set for Aug. 17. His attorney said there was no agreed-upon sentence.

Morel, who is free on $50,000 bond pending sentencing, served as district attorney from 1979 to 2012. After opting not to seek re-election, Morel served as an assistant prosecutor under his successor for several months before retiring amid the FBI investigation.

The Mississippi River bisects St. Charles Parish, which is about 20 miles west of New Orleans and has roughly 50,000 residents.