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A former FBI special agent gave an inside look into how federal and state law enforcement apprehended the El Salvador migrant suspect in the multi-state homicide investigation of Rachel Morin.

Retired FBI Supervisory Special Agent Scott Duffey told Fox News Digital that, at the beginning of any homicide investigation, law enforcement starts with those closest to the victim.

"First and foremost, you start with loved ones, whether it be a spouse, a partner or a romantic partner," he said. "And then once you exclude them, then you move outward."

Duffey, who met with Morin's family in Harford County, Maryland, after her murder, said the FBI's Combined DNA Index System (CODIS) allowed law enforcement across state lines to stay on the same page as the 10-month investigation progressed.

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Victor Martinez Hernandez and Rachel Morin

Victor Martinez Hernandez, 23, was arrested in the murder of Rachel Morin. (Harford Co. Sheriff's Office)

CODIS is a database used by federal and state law enforcement to compile DNA profiles of convicted offenders, unsolved crime scene evidence and missing persons.

A break in Morin's homicide case came after a piece of DNA evidence showed up thousands of miles from Morin's Maryland hometown in Los Angeles weeks after her disappearance.

Scott Duffey speaking to Fox News

Scott Duffey, a retired former FBI special agent, discussed Rachel Miron's homicide with Fox News Digital. (Fox News Digital)

"And lo and behold, the two DNA matches from DNA from Rachel Morin's crime scene and the DNA from LA were a match through CODIS," he said.

Duffey said the CODIS operates under "strict guidelines," which gave him comfort about the legitimacy of the DNA match. 

"They have very strict guidelines, and the fact that you had two DNA matches was very comforting to me as a former investigator," he said.

Missing woman Rachel Morin smiling

Rachel Morin went missing after taking a walk on a Maryland trail.  (Harford County Sheriff's Office/FB)

The former special agent said the unexpected twist came after investigators discovered Morin's suspected killer, later identified as Victor Martinez Hernandez, an illegal El Salvador migrant, was not a U.S. national.

Martinez Hernandez's immigration status prompted law enforcement to rely on FBI international offices, called legal attachés.

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"What's their next stop out of the country into El Salvador?" Duffey said. "Now, it's one thing when you have a genealogical tree, and you're able to say, 'OK, we're getting closer' and the DNA experts are able to say, ‘Hey, this is a match’.

"But when you go out of the country, you lose certain ownership of an investigation."

A photo of Rachel Morin is posted on a tree along a hiking route

A photo of Rachel Morin was posted to a tree by her family along the Ma and Pa Trail in Bel Air, Md., Aug. 10, 2023. The 37-year-old was murdered while hiking the trail. (Mega for Fox News Digital)

The FBI international office in El Salvador was used to "bridge that gap" with law enforcement in the U.S. on Morin's case," Duffey said.

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"But having an FBI office in El Salvador, they're able to bridge that gap with American law enforcement and with El Salvadorian authorities," he said.

Victor Martinez Hernandez in a t-shirt

Victor Martinez Hernandez, 23, was arrested in the murder of Rachel Morin. (Harford Co. Sheriff's Office)

El Salvador authorities then communicated with U.S. law enforcement that Martinez Hernandez had fled his country of origin after brutally killing someone in the Central American country.

The news prompted the FBI to put out a "red notice," which told all law enforcement involved in Interpol to be on the lookout for Martinez Hernandez.

"That red notice would have alerted authorities, ‘Do not let this individual go any further. Stop and detain,'" Duffey said. "All those things are falling into place, not to mention the hard work that Harford County investigators pursued with LA authorities."

Rachel Morin headshot next to a photo of her hadn her boyfriend wearing a baseball cap

Rachel Morin, a mother of five, was found dead on a hiking trail. She was reported missing by her new boyfriend, far right, Richard Tobin. (Facebook)

Duffey said he believes law enforcement used Martinez Hernandez's "digital forensics footprint" to piece together the final touches on its investigation.

"A cellphone, an email address, something that this individual is communicating with anybody else and thereby having such a footprint that law enforcement can zone in, pinpoint to a relatively small, very tight area to where surveillance could go out and investigate," he said.

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Duffey said it was "amazing" Martinez Hernandez, a wanted fugitive, managed to evade capture for 10 months after Morin's homicide.

"It will be interesting to see what his means were," he said. "Who was supporting this individual who was already a fugitive from justice from his country of origin?"

Rachel Morin in black bikini

Richard Tobin, far left, denied he had any role in his girlfriend Rachel Morin's death after her body was found on a Maryland hiking trail. (Facebook)

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On Saturday, Harford County Sheriff Jeffrey Gahler announced the arrest of Martinez Hernandez.

Gahler said the 23-year-old was arrested in Tulsa, Oklahoma, and charged with first-degree murder and first-degree rape.

The 23-year-old migrant illegally crossed into the United States in February 2023, police announced.

"We all suspected that Rachel was not his first victim," Gahler said. "It is my understanding that this suspect, this monster, fled to the United States illegally after committing the brutal murder of a young woman in El Salvador a month earlier, in January of 2023."

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Gahler said a first DNA match for Martinez Hernandez was from a Los Angeles attack in March 2023.

"Once in our country, and likely emboldened by his anonymity, he brutally attacked a 9-year-old girl and her mother during a home invasion in March of 2023 in Los Angeles," Gahler said. "And as everyone I believe is aware, that was our first DNA match linking Rachel's case to the one in Los Angeles."

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Morin, 37, was reported missing in August by her boyfriend, who said she never returned after going out for a run on the Ma & Pa Trail, a pedestrian trail in Bel Air, a quiet and typically safe town about 28 miles northeast of Baltimore, Aug. 5, 2023.

Her body was found on a trail the next day.