PHOENIX (AP) — Police in the Arizona border city of Nogales have been told to carry guns when they aren't at work after smugglers threatened to retaliate against two off-duty officers who stopped a vehicle carrying 400 pounds of marijuana.

Nogales Police Chief Jeff Kirkham said Tuesday that he considers the threat credible because various informants were able to identify the specific officers in the city of 20,000 who thwarted the drug load earlier this month.

"Are they credible? Yes," Kirkham said. "Are they going to carry through with those threats? I don't know."

Kirkham said the U.S. Border Patrol has sent additional agents to the area east of the city where the off-duty officers, who were riding horses, stopped the smugglers during the first week of June.

The smugglers left behind their vehicle and fled into Mexico, while the two armed officers guarded the vehicle and its drugs.

No one was injured in the bust, and there hasn't been any retaliation against officers.

Nogales police managers advised off-duty officers to carry their guns, avoid the part of the desert where the bust was made, and carry a cell phone or police radio to report retaliation attempts or anything suspicious.

Kirkham said smugglers view off-duty officers differently than uniformed officers. "They believe that when you don't have a uniform on, you are not a cop anymore, that you are a civilian who doesn't take action on felony crimes," he said.

Border Patrol spokesman Omar Candelaria declined to say how many additional agents were sent into the desert area where the bust was made.

The police chief said he believes this is the first threat made by smugglers on an officer in his city and that his city doesn't have the cartel murders that are seen in communities south of the border.

Two weeks ago, two Border Patrol agents shot and wounded two suspected drug smugglers about 40 miles west of Nogales after agents said the traffickers threw rocks at them.

The neighboring Mexican city of Nogales has had attacks on police officers since the Mexican government started its war on the country's powerful drug cartels.

A deputy police chief in Nogales, Mexico, and his bodyguard were fatally shot in March when gunmen in a pickup truck opened fire on them with assault rifles.

In November 2008, the police chief for the Mexican state that includes Nogales was fatally shot as he entered a hotel with his bodyguard and other officers. The gunmen used grenades as diversions in the fatal ambush a few miles south of the Arizona border.

The threat was first reported by the Nogales International.