Leaders across the country felt increasing pressure on Friday to shut down protest encampments after two men died in shootings and another was found dead from a suspected combination of drugs and carbon monoxide poisoning caused by a propane heater inside a tent. Citing a strain on crime-fighting resources, police officers pleaded with Occupy Oakland protesters to leave their encampment at the City Hall plaza where a man was shot and killed late Thursday. The Oakland Police Officer’s Association issued an open letter saying the camp was pulling officers away from crime-plagued neighborhoods. “Please leave peacefully, with your heads held high, so we can get police officers back to work fighting crime in Oakland neighborhoods,” the letter said. Late in the afternoon, police officers acting at the direction of Mayor Jean Quan distributed fliers to protesters warning that the camp was in violation of the law and had to be disbanded immediately. The notices warned campers they would face arrest if tents and other materials were not removed, although the warnings did not specify a deadline. The Oakland shooting occurred the same day a 35-year-old military veteran apparently shot himself to death in a tent at a Burlington, Vt., Occupy encampment. On Friday, a man was found dead inside a tent at the Occupy Salt Lake City camp from what police said was a combination of drug use and carbon monoxide.