An unoccupied house caught fire Sunday afternoon on the near west side of Janesville, the second time the same house has been on fire this year.

A 12-year-old boy is believed to have started the fire at 121 S. Academy St. while he and another boy were playing with lighters.

The boy was referred to juvenile authorities Sunday afternoon, said Lt. Charles Aagaard of the Janesville Police Department.

The second boy, 13, was not arrested.

The boys were not inside the structure. The fire broke out on a porch of the house, Aagaard said.

A witness saw the boys there, which helped investigators find them, Aagaard said.

Firefighters responded at about 3:15 p.m. Sunday to find fire traveling throughout the two-story house, according to a fire department report.

Flames were visible on the outside of the house and caused damage to the interior. No water, electric or natural gas utilities were connected to the building at the time of the fire, said fire department Battalion Chief Ryan Murphy.

The estimated damage to the structure could not be determined because of damage from a fire April 1, according to a fire department news release.

The Janesville Police Department and Rock County Sheriff’s Office are conducting a joint investigation into the cause of the fire.

Aagaard said the sheriff’s office was asked for help because of staffing issues at the police department.

Two residents were displaced in the April fire.

The vacant house also was broken into in May.Anyone with information is asked to contact the police department at 608-757-2244 or the sheriff’s office at 608-757-8000.