Investigators have made an arrest in the mysterious killing of a young woman whose body was found dumped on the side of a road in San Diego's North County more than one month ago.

San Diego County Sheriff's Department (SDSO) Lt. Kenn Nelson confirmed an arrest was made late Tuesday in connection with the homicide of Antonia Herrera, 23. Nelson said Herrera was shot to death.

Suspect Paul Castro, 27, a resident of Las Vegas, was taken into custody by homicide detectives with the Las Vegas Metropolitan Police Department. He’s a documented gang member, authorities said.

Castro will be extradited back to San Diego to face charges in the murder of Herrera. He was an acquaintance of the victim; Nelson said they met through mutual friends in Las Vegas.

Herrera's body was found on Jan. 12 on the side of the 27600 block of Champagne Boulevard in unincorporated Escondido, near Interstate 15.

Her manner of death was ruled a homicide, but her autopsy was sealed. Nelson said Wednesday that investigators believe Herrera was shot to death inside a friend’s car as she was traveling back to San Diego from Las Vegas on Jan. 12. Castro was in that car, too.

“We do have that vehicle in our possession, and we will be processing it, based on search warrants that we have obtained for that,” Nelson said.

The lieutenant said investigators have not yet determined a motive for the murder.

Castro is believed to be the only suspect in the killing. Other people were in the car, but Nelson said they are not suspected in the shooting of Herrera.

"We believe Mr. Castro is solely responsible for her death," Nelson added.

Herrera's sudden death left the young woman's family searching for clues, trying to figure out who was the last person to see her alive.

In an interview in January, Herrera's family told NBC 7 the victim, in her last few months alive, had feared for her life. She was so concerned, she even took out a life insurance policy on herself just a few months before her death.

"Someone has to know something," said Tinamarie Herrera, the victim's sister.

Eliza King, Herrera's roommate at San Pasqual Academy, told NBC 7 she had seen Herrera about a week before her death, during classes for an internship program.

"She was working on getting her life together," King told NBC 7 in January.

King said Herrera told her she was staying with a male friend for the week so she could have a way to get to the program. King said Herrera told her she felt uncomfortable staying with the friend, but it was only for a short time.

King said Herrera was beloved.

"We valued her, people love her and it's just not fair," she said.

Nelson hopes the arrest in this case helps Herrera's family find "just a little piece of solace."

NBC 7 spoke with King again on Wednesday. She said this is just the beginning of seeking justice for her friend.

“To go and see your friend in a casket – that you’re used to seeing so lively and so happy all the time is really hard,” King said, fighting back tears. “It’s really hard to deal.”

King said it’s especially difficult to imagine her friend being dumped on the side of a road like garbage, as if she was “nothing.”

“It’s really hurtful, it hurts, not just me, but my soul like to the core. You don't do that to somebody, like they are trash,” said King. “She wasn’t trash.”

Herrera was originally from Chula Vista but spent time in transitional group homes all over San Diego, including in National City, Oceanside and her most recent group home in San Marcos.

King said she often visits the site where Herrera’s body was found. A makeshift memorial has been set up there, with balloons and candles.

“Every time I go, I stand there, looking and I’m like, ‘How could somebody just do that?’ There's barbed wire there so, somebody just tossed her over there like she was nothing,” she added. “It's really painful.”