SAO PAULO, BRAZIL - Gunmen, possibly from one of Brazil's most notorious organized crime groups, this weekend killed the third off-duty corrections officer in less than a week.

Police are investigating whether the killings were ordered by the gang -- the First Capital Command -- which in May orchestrated an unprecedented wave of violence that rocked South America's biggest city.

The officers were gunned down less than a week after police killed 13 suspected gang members who planned to attack COs, police said.

Last Monday, 12 men and one woman suspected of belonging to the gang, better known by its Portuguese initials PCC, were killed in a gunfight with police outside a prison in Sao Bernardo do Campo, an industrial suburb on the outskirts of Sao Paulo.

Acting on orders from imprisoned PCC leaders, they planned to shoot as many as 60 officers from four lockups over a 10-day period as they headed to work or finished their shifts, according to district police chief Marco Antonio de Paula Santos.

Officers learned about the plan by listening in on gang leaders' phone calls and from informants.

Also on Sunday, police arrested 14 suspected PCC members in Peruibe, 81 miles southwest of Sao Paulo. The 14 were arrested as they were heading to a meeting, apparently to organize attacks against corrections officers and police, the Sao Paulo State Public Safety Department said.

In May, imprisoned gang leaders allegedly ordered attacks against police across the city and Sao Paulo state, touching off a weeklong wave of violence that killed nearly 200 police, corrections officers, suspected criminals and jail inmates.

The gang leaders allegedly ordered the attacks because of anger over a government plan to transfer gang leaders to more secure prisons.