When Jefferson Parish deputies discovered their colleague, Sgt. Tracey Marshall, had been fatally shot in front of her Terrytown condominium this week, the woman’s estranged boyfriend quickly emerged as the prime suspect.

James Darby was booked Tuesday in Tuscaloosa, Alabama, on a charge of second-degree murder in Marshall’s death, accused of shooting the woman and then fleeing to Tuscaloosa, where he was arrested.

Darby, an ex-convict from New Orleans, dated Marshall for several years in what Jefferson Sheriff Newell Normand described as a “very tumultuous relationship.”

In 2011, Darby violated a protective order Marshall had taken out against him, the sheriff said. As recently as last month, Darby lodged a complaint against Marshall, claiming she struck him with her police baton.

The arrest brought a swift conclusion to a murder investigation that hit close to home for the Sheriff’s Office, an agency Marshall had worked for since 2005. She was assigned to the Jefferson Parish Correctional Center until February, when she transferred to the courthouse to work as a bailiff.

Normand told reporters Wednesday that the sergeant had been “very well liked by everyone she touched in this organization.”

“We’re a family here, not unlike most other police organizations, so when you lose one of your own, whether in the line of duty or not in the line of duty or just by natural causes, there’s a sense of loss to us,” Normand said. “Obviously, she will be significantly missed.”

The sheriff said investigators are still piecing together the particulars of what may have motivated the shooting and whether Marshall had known the full extent of Darby’s criminal history, a record that included a conviction for aggravated incest. But he portrayed the recurring domestic strife as a critical element of the case, one that prompted detectives to trace Darby’s cellphone data.

A similar query allowed the authorities to pinpoint Darby’s whereabouts in Tuscaloosa, a city where he had lived for a time after Hurricane Katrina.

Darby offered a “real sketchy” account of his whereabouts Sunday evening, Normand said, and denied being in the area at the time of the shooting before changing course and admitting he had not only followed Marshall but was, in fact, in front of her condo for a period of time. Still, he disavowed any involvement in the shooting.

“He’s changed his stories quite a few times,” Normand said, adding that cellphone data put Darby in Terrytown between 9:35 p.m. and 9:47 p.m.

Marshall had been on a date Sunday evening, the sheriff said, and phone records show she had been talking with the same friend between 9:29 p.m. and 9:39 p.m.

The Sheriff’s Office received a report that shots had been fired about 9:40 p.m. but did not find the victim until about 12 hours later, when a passing neighbor spotted Marshall’s hair hanging out of the shattered driver’s side window of a red pickup in the 2300 block of Ashley Drive.

Darby has “attempted to secret the fact that he was convicted” of incest, Normand said, and has been arrested several times for failing to register as a sex offender.

“We have talked to several friends of his that knew he had served time but never knew that he was a registered sex offender,” Normand said. “At this time, we don’t know for certain one way or the other whether Tracey had any idea that he had been arrested for that. However, we do believe that she was aware that he may have had a criminal record.”

Extradition proceedings were said to be underway to return Darby to Jefferson Parish.

Even though Marshall was a law enforcement officer, Normand said her death is not considered a capital offense on its face because she was not on duty at the time of her death.

Follow Jim Mustian on Twitter, @JimMustian.