SEDONA, Ariz. (AP) – The owners of a resort where two people were overcome in a sweat lodge and later died are asking for prayers in hopes that something positive will come out of what they say was a tragic and unexpected accident.

Angel Valley Retreat Center owners Michael and Amayra Hamilton said in a statement Tuesday that a prayer ceremony has been conducted at the sweat lodge site for the victims, and their friends and families. They say a heart-shaped memorial has been laid with stones for 38-year-old Kirby Brown of Westtown, N.Y., and 40-year-old James Shore of Milwaukee who died Thursday evening.

Nineteen other people were treated for symptoms ranging from dehydration to kidney failure. On Tuesday, one remained in critical condition, another was in fair condition and a third was in good condition.

Sweat lodges – dome-shaped structures made of wood, animal skins, canvas and other materials – are commonly used by American Indian tribes to cleanse the body and prepare for hunts and ceremonies. Stones are heated up outside a lodge, brought inside and placed in a pail-sized hole. The door is closed, and water is poured on the stones, producing heat aimed at releasing toxins in the body.

The Hamiltons had rented the facility just outside Sedona to self-help expert James Arthur Ray for his five-day “Spiritual Warrior” retreat. Fifty-five to 65 people attending the program were crowded into the 415-square-foot sweat lodge during a two-hour period Thursday night.

The Yavapai Sheriff’s Office is investigating the two deaths to determine if criminal negligence played a role in the deaths or illnesses. No charges have been filed.

Tests for contaminants ruled out carbon monoxide poisoning as a cause. Autopsies on Brown and Shore were conducted, but the results are being withheld pending additional tests.

Investigators say the two-hour session inside the sweat lodge consisted of eight 15-minute rounds and various spiritual exercises led by Ray. After each round, the flap to the crudely constructed structure was raised to allow more heated rocks to be brought inside.

Authorities said participants were highly encouraged but not forced to remain in the sweat lodge for the entire two hours. The participants had fasted for 36 hours as part of a personal and spiritual quest in the wilderness, then ate a breakfast buffet Thursday morning.

Officials say the sweat lodge, built specifically for the five-day retreat, lacked the necessary building permit.