Ryan Kilian was exchanging texts with his girlfriend, Jena Christiansen, on Monday afternoon when suddenly the messages stopped.

He soon learned the reason why: Jena and her mother, Karen, were killed when an 800-pound boulder tumbled from a truck passing on the other side of the road and smashed into their vehicle.

Both women were pronounced dead at the scene in Rosemount. The truck was nowhere to be found.

On Wednesday night, police located who they say was the driver at an Inver Grove Heights home. The suspect was being held Friday in Dakota County jail and has been charged.

"It's just tremendously sad for the family," said Rosemount Police Chief Mitchell Scott. "This is something that could have been prevented."

A representative of the Christiansen family said they weren't ready to talk. But in a series of messages Thursday, Kilian said he was heartbroken.

"How does this happen / Jena Christiansen, you are my world / Wake me up from this nightmare," he wrote on Facebook Wednesday.

Kilian said that both women were "loving people," and that Karen Christiansen had been kind and welcoming to him. Jena was quirky and sarcastic, he said, often making him laugh so hard he cried.

Jena, 32, had been living in Superior, Wis., and working as a Red Lobster restaurant manager. Karen, 67, was retired from her job as a training representative at Dakota County Technical College (DCTC) in Rosemount.

Kilian said that he and Jena were in the process of moving to the Twin Cities, where she had just started managing the Red Lobster in Bloomington. "All her employees loved her," Kilian said.

At DCTC, Karen had helped students seeking job training and companies that wanted their employees to learn new skills. "She was a very sweet person and such a hard worker," said Marlo Teal, a co-worker and friend, in a message.

"She was so patient with students and helped a lot of people."

Teal added that she was close with her family, including Jena: "There was a lot of love."

On Monday, Karen Christiansen had gone to lunch with some friends. Jena picked her up after she got off work.

Karen moved into the driver's seat and the two headed toward the family home in Shoreview.

Police said they were interviewing witnesses and finalizing the investigation following the suspect's arrest. Scott said that the suspect declined to give a statement when he was arrested, saying only that he wanted an attorney.

Scott said it wasn't clear whether the suspect was aware that the boulder had fallen out of the truck or that it had caused a collision. But the police chief said he had his suspicions.

"We feel that he knew, but … we don't know what he was thinking," he said.

Scott said the accident was rare but that it's not unusual for people to improperly secure their loads.

He recalled a man who died on Interstate 494 several years ago after a tire busted through his windshield.

Scott estimated that the boulder that caused the accident measures about 3 feet by 3 feet and weighs about 800 pounds.

Kilian posted a video of himself with Jena on Facebook Wednesday, writing above it: "I miss her."

He said she was a free spirit and didn't care what other people thought. "She just wanted to be happy," he said.

Erin Adler • 612-673-1781