It was spring 1973, when hitchhiking was common practice for young people with somewhere to go and no car to drive there.

Maggie and Mary Jenkins were sisters, ages 18 and 16. The former North Scranton (Pa.) Junior High School students' journey to Key West was supposed to be a carefree break soaking in the Florida sun. They never returned, and their deaths remain a mystery.

"The Keys, especially in the 1970s, were very transient," said Detective Mark Coleman of the Monroe County Sheriff's Office. "Two teenage girls, hitchhiking.... You can imagine the dangers."

They were found dead together in an isolated, wooded area of Key Largo on May 3, 1973, about 100 miles from Key West off U.S. 1.

"They were my best friends," their older sister, Sarah Jenkins Barlow, said from her home in Wilmington, Mass. "Every place we went, we went together.

"I wonder what would have happened if I [had gone] with them," said Barlow, who was 19 when she learned her sisters had been murdered.

Coleman was given the job of trying to piece together the crime, considered a cold case, several years ago.

The sisters were last seen hitchhiking a few miles from Key West, apparently trying to get back home, the detective said.

Flipping through the case file, he learned that Maggie and Mary were shot multiple times and it appeared they had been sexually assaulted. A man walking his dog found their bloodied bodies covered with sticks and leaves.

Last year, investigators armed with the latest technology took a closer look at specks of male DNA found on the sisters' bodies.

"We do have some workable and viable evidence that is being looked at right now," Coleman said, although the DNA is very degraded, making it difficult to link to a suspect.

Due to some rough times for the family during their younger years, Maggie and Mary spent many years living in Scranton, where they were raised by aunts and uncles.

Barlow remembers visiting her sisters in Scranton, where they would talk the day away about "boys they had crushes on." Mary would show off awards she won in gym class, while Maggie loved horses.

"She used to say she was going to have a farm and have hundreds and hundreds of horses," Barlow said.

Investigators have struggled to solve the case — which remains open — because of where the sisters were killed and because they were hitchhiking, making them easy targets for random strangers who could easily flee, Detective Coleman said. Because U.S. 1 is the only highway connecting the Florida Keys to the mainland, it is very busy with tourist traffic.

In 2007, a suspect from St. Louis voluntarily submitted a sample of his DNA, but it did not match what was found on the girls, Coleman said.

DNA recovered from the sisters' bodies was recently sent to a state crime lab and reviewed by the FBI to see if it could be linked to any convicted serial killers. Ted Bundy, who went on a killing spree of young women in Florida the 1970s, was eliminated as a suspect. So was Christopher Wilder, another serial killer who targeted women in Florida.