OAKLAND, Calif. -- Leaders across the country felt increasing pressure Friday to shut down Occupy 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 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, which represents rank-and-file police, issued an open letter saying the camp is pulling officers away from crime-plagued neighborhoods.

"With last night's homicide, in broad daylight, in the middle of rush hour, Frank Ogawa Plaza is no longer safe," the letter said. "Please leave peacefully, with your heads held high, so we can get police officers back to work fighting crime in Oakland neighborhoods."

The sentiments were echoed by Mayor Jean Quan.

"It is another example of why we need to peacefully close the encampment at City Hall," she said of the shooting. "We are asking everyone at the plaza to leave."

Quan said the city would issue another official notice to protesters that they must leave the camp. However, she did not give them 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 encampment, from what police said was a combination of drug use and carbon monoxide.

The Oakland killing has further strained relations between the anti-Wall Street protesters and local officials, many of whom have called on the demonstrators to leave.

A preliminary investigation into the gunfire suggested it resulted from a fight between two groups of men at or near the encampment, police Chief Howard Jordan said. Investigators do not know if the men in the fight were associated with Occupy Oakland, he said.

Protesters said there was no connection between the shooting and the camp.

Salt Lake City police Chief Chris Burbank said officers have made 91 arrests at the camp, roughly the same number seen in the area during all of the last year.

Tensions were also high at the 300-tent encampment in Portland, Ore., which has become a hub for the city's homeless people and addicts.

Mayor Sam Adams ordered the camp shut down by midnight tonight, saying the tipping point came this week with the arrest of a camper on suspicion of setting off a Molotov cocktail outside an office building, as well as two non-fatal drug overdoses at the camp.

"I cannot wait for someone to die," he said.

Many at the camp said they would resist any effort to remove them.

Police said some elements inside the camp may be building shields and makeshift weapons, including nails hammered into wood, while trying to gather gas masks.