A clash between rival gangs was responsible for fights that began Monday in the recreation yard at the Oregon State Penitentiary and erupted again two days later, the state Corrections Department said.

SALEM — A clash between rival gangs was responsible for fights that began Monday in the recreation yard at the Oregon State Penitentiary and erupted again two days later, the state Corrections Department said.

Three prison employees and an unknown number of inmates were treated for cuts and bruises. About 100 inmates have been put into segregation or transferred to other prisons, The Oregonian (http:is.gd/jJP0PA ) reported.

The maximum-security prison that holds more than 2,000 inmates was in a modified lockdown that could last through the weekend. No visitors are allowed during the lockdown.

Brian Belleque, a state Corrections Department administrator, said investigators are still trying to determine what caused the fights. Several broke out in the recreation yard about the same time, suggesting they were "not spontaneous. ... There was some underlying tension among the two groups," he said.

Belleque, citing security reasons, wouldn't identify the gangs, which the agency calls "security threat groups."

Guards fired four warning shots during the Monday fighting.

One-on-one fights between gang members erupted Wednesday in the prison dining hall, showers and housing units, prompting prison officials to slow the return to normal operations, Belleque said.

In April, large fights broke out over three days at the penitentiary. Those fights involved other rival gangs of white supremacists and Latinos, he said.

Investigators said the violence started after an inmate went into the cell of another and kicked him in the head without provocation. That triggered a series of fights, escalating in size, prison officials said.

A tower guard fired a warning shot to break up one of the fights. The agency later found that use of force justified.