According to WLOX in Biloxi, Panks was driving his Chevy Silverado west on the interstate when an eastbound Toyota Camry crossed the median and smashed into his truck. The driver of the Camry, Ross Labrosse, 36, of Violet, was pronounced dead at the scene, NOLA.com reported.
Panks was pronounced dead about an hour after the crash, at Garden Park Medical Center in Gulfport.
Both men died of blunt force trauma, WLOX reported.
Harrison County Coroner Brian Switzer told NOLA.com there were reports of a car matching the description of Labrosse's Camry traveling erratically on the interstate prior to the crash. He said Labrosse was apparently trying to pass another vehicle when he lost control.
"That's as innocent a victim as you have," Switzer said of Panks.
Three passengers in Panks' pickup truck were taken to area hospitals for treatment. WVUE in New Orleans reported that the passengers included Panks' brother, Jared Panks, and two close friends.
All three remained hospitalized Monday, the news station said. Lemonier wrote on Facebook that their injuries were “extensive.”
Lemonier, who had an engagement photo set as her profile picture, mourned her fiancé on social media.
"My heart has been shattered into a million pieces. I still can't believe this is real," Lemonier wrote. "I love you so much, Jonathan. You forever have my heart."
She wrote that Panks was the best thing that had ever happened to her.
“I am so grateful that you chose me to be your forever,” she wrote. “I love you with everything I have.”
Lemonier, who was set to marry Panks on Nov. 23 in Kentwood, told WVUE he was the love of her life.
"He was the best. He's the best person you could ever meet," Lemonier told the news station. "He has so many friends and so many people that loved him."
Lemonier said she was at a family gathering when one of Panks’ friends called her from the scene with the tragic news. The friend told her Panks, who was not conscious, had to be cut from the wreckage of his truck.
After he died, some of Panks' fellow sheriff's deputies traveled to Mississippi to escort their friend and colleague's body home, WLOX reported.
Panks' older sister, Melissa Panks, told WVUE she was flying back from Colombia when she learned of the crash.
"I felt pretty helpless and like I didn't have any control, because I couldn't be there for my family," Melissa Panks told the station. "And then, when I got the other phone call, I was in the middle of the plane in my seat and just was in shock. And everybody around me heard, and I just started crying."
Jonathan Panks and Lemonier had been together for nearly five years. Her father, David Lemonier, said he feels as though he lost a son in the accident.
"Jonathan was part of this family for almost five years," David Lemonier said. "So, he's like a son. He is a son."
St. Tammany Parish Sheriff's Office officials said deputies are mourning their colleague, who had been with the agency since 2006. Sheriff Randy Smith told NOLA.com the news was difficult to accept.
"Jonathan was much more than a dedicated deputy who served our community with pride and professionalism," Smith said. "He was a good-hearted, can do-spirited friend who loved the law and cared deeply for the people and parish he served."
Nick Tranchina, a former deputy, said he mentored Panks when they were on the SWAT team together.
"He was a good guy, top-notch," Tranchina told the news site. "It's a hard loss for the community and the department."
In a statement posted on Facebook, Sheriff's Office officials said Panks had worked in corrections, criminal patrol and street crimes and was currently assigned to the marine division. Panks helped recover the body of his cousin, Chad Michael Panks, 33, of Pearl River, last summer following a boating accident on the West Pearl River.
WVUE reported at the time that Chad Panks and his dog were traveling to his houseboat around 2 a.m. June 15, 2018, when the accident apparently took place.
Family members realized something was amiss when the dog returned to Chad Panks' home that afternoon without him, the news station said. They found his sunken boat an hour later and sheriff's deputies, along with agents from the Louisiana Department of Wildlife and Fisheries, located Panks' body in the water around 9 p.m. that day.
Chad Panks' sister, Shannon Panks Torres, wrote on Facebook on Sunday that words could not describe the pain she felt over her cousin's death.
"You found Chad when we needed you the most in the world, and now I just want to be home to hold Colette, Melissa, Jason, Jared and Aunt Lori for you," Torres wrote. "You will be forever missed, never forgotten and loved beyond measure. I love you to the moon and back."
Numerous friends and family members shared Panks' official Sheriff's Office photo as their profile pictures in his memory. One relative shared a photo showing Panks holding her newborn.
Vince Coulon shared a photo of the American flag outside his home at half-staff.
"For Jonathan," he wrote.
Brett Nabors, who works in enforcement for the Department of Wildlife and Fisheries, also posted Panks’ photo.
"Rest in peace, brother," Nabors wrote.
Lemonier told WVUE it has been difficult to stay in the home she shared with her fiancé. She recalled how he would often leave her little notes around the house, including one he left Thursday morning before leaving for his weekend bachelor's getaway.
"He said, 'Babe, dishes in the dishwasher will be clean, trash is out. I want you to know I will truly miss you the few days I'm gone. You're the best thing that ever happened to me," Lemonier read for the station, fighting back tears. "I love you, and I hope you have some fun this weekend, too."
Now, instead of continuing to plan their wedding, Lemonier and Panks’ family are planning his funeral.