NORTH SIOUX CITY - Authorities say a Sioux City teen injured her legs when she fell at least 15 feet off the side of a railroad trestle trying to retrieve a cell phone.

Police are investigating the incident, which happened late Thursday at the midway point of the 300-foot train bridge spanning the Big Sioux River from Sioux City's Riverside neighborhood to North Sioux City.

North Sioux Police Sgt. James Heeren said Eva Barron, 17, and some friends were walking across the bridge when one of them dropped a cell phone into the mud at the base of one of the bridge's support pillars.

Barron attempted to slide feet-first down the concrete pillar using a makeshift tether of coaxial cable and an electrical cord, Heeren said, but the tether broke.

"The pillar was a 30-foot drop, but witnesses said she was halfway down before the makeshift rope gave out," Heeren said.

The accident is under investigation, Heeren said, because it happened on private property posted with no-trespassing signs.

"It's a very unsafe situation, because you don't know when trains are coming," he said.

Railway officials were notified of the situation and asked to prevent trains from approaching the bridge.

North Sioux City Fire Chief Bill Pappas said firefighters rappelled down the side of the bridge to Barron, attended to her immediate medical needs and loaded her into a wire basket.

Other firefighters hoisted the basket back onto the bridge and then carried Barron to a waiting vehicle.

She was taken to Mercy Medical Center - Sioux City for treatment of what are believed to be lower-leg injuries. Tonight, a hospital official said Barron had been treated and released.