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A 45-year-old geologist from Edmond, Okla., collapsed and died while running Sunday’s LaSalle Bank Chicago Marathon, the second fatality in the race’s 23 year history.

The man, Danny Towns, had been running for slightly more than four hours when he collapsed around noon at the 22.5-mile mark near Comiskey Park , police said.

A doctor and nurse running behind Towns administered CPR until an ambulance arrived, but efforts to revive him at the scene and at Mercy Hospital and Medical Center were unsuccessful.

Two years ago, Kelly Barrett, 43, a pediatric dentist from Littleton, Colo., died of hyponutremia, or low sodium levels, apparently related to drinking too much water during the 26.2-mile race.

Race officials said Towns apparently went into cardiac arrest, but what caused it was unknown. An autopsy is set Monday by the Cook County medical examiner.

Towns’ wife, Debbie, told race officials her husband, an experienced runner, was in apparently good health and had run a marathon last December.

The couple had three children: two girls in 8th and 6th grades and a son in 4th grade, neighbors said. Towns frequently ran with the two youngest children. This past year he had been seen running with his son through the streets of Edmond, a suburb just north of Oklahoma City. The two were training for a race in Tulsa that they planned to run together, Towns’ neighbor Bernice Kennedy said.

“Before they left they told me that running was something he liked to do and they all liked to go and be together,” Kennedy said.

“We all saw him running all the time,” said neighbor Melissa Pitts, whose children play with the Towns’ children.

Towns worked at Louis Dreyfus National Gas Co. in Oklahoma City. Dreyfus officials and Debbie Towns could not be reached for comment.

Neighbors said Towns was not the kind of runner who would push himself past extremes.

“If he knew he was in trouble he would have stopped,” next door neighbor Sue Laurick said.

In the death two years ago, doctors said swelling in Barrett’s brain blocked her respiratory impulses and led to cardiac arrest. The swelling was the result of fluid retention caused by a drop in Barrett’s blood-sodium levels caused by the rare case of water intoxication.

The medical staff for this year’s marathon was more than 300, including at least 40 medical doctors along the course, 10 more than last year, officials said. There were 17 aid stations and 35 ambulances on the course, and two medical tents at the finish line.