The gunmen in three vehicles drove up to the car wash in the city of Tepic and opened fire without provocation, said Fernando Carvajal, public safety secretary of Nayarit state, where the city is located. Fifteen men were killed and three people were injured.
The motive was not immediately clear but investigators suspect it was the work of organized crime, Carvajal told reporters.
He said most of the victims were recovering drug addicts and worked at the car wash. One victim, however, had just driven up to the business in a motorcycle and appeared not to have worked there, and another body was found at a nearby fruit stand.
Carvajal said the owners of the business have another car wash in the city where a man was killed Tuesday, and police were investigating whether the attacks were linked.
Nayarit Gov. Ney Gonzalez said investigators believe some of the victims had been washing a stolen car.
"These boys were fighting for hope, were fighting against drugs," Gonzalez said in a statement posted by the state government. "The same as in Ciudad Juarez, the same thing in Tijuana," he said, referring to recent attacks on rehab centers in those cities.
President Felipe Calderon, speaking at a forum on security, called for a minute of silence for the victims of the Tepic attack and two other massacres that have occurred since Friday: an attack on a birthday party that killed 14 young people in the border city of Ciudad Juarez, and a shooting at a drug rehab center in Tijuana that killed 13 recovering addicts.
Prosecutors in Tijuana identified one of those killed in that attack as a Colombian man, Wilson Ramirez Pena, 42. They did not say what he was doing in Tijuana.
The three attacks did not appear to be related. Such mass shootings have become increasingly common in Mexico, where drug-gang violence surged after Calderon launched an offensive against drug cartels soon after taking office in December 2006.
In an interview with the BBC released Wednesday, Calderon said he had to launch the offensive in part because his predecessor Vicente Fox -- who served as president from 2000 to 2006 and is a member of Calderon's conservative National Action Party -- "didn't act in time" to stem the rise of the cartels.
"I have a great respect for former president Fox," Calderon said. "But I think he made a lot of mistakes on this issue. Perhaps the most important was not acting in time on this.
"I think that if Mexico had started to fight against this problem 10 years ago, we would be talking about something completely different now."
Cartel-style violence has picked up this year in Nayarit, a small Pacific coast state wedged along drug trafficking route disputed by several drug gangs.
In April, 12 bodies, eight of them partially burned, were found in the fields outside the Nayarit town of Xalisco. Gonzalez, the governor, ordered schools to close early in June because of rising violence.
Drug gangs were blamed in the first two massacres.
In Tijuana, prosecutors say they are investigating whether the attack there was related to a record seizure of nearly 135 tons of marijuana last week. Shortly after the attack, a voice was heard over a police radio frequency threatening that there would be as many as 135 killings in Tijuana -- a possible reference to the government's pot haul.
In Ciudad Juarez, investigators said two men found dead Tuesday -- one of them decapitated -- might have been involved in the birthday party massacre Friday night. A note left with the bodies accused the men of killing women and children. The victims of the party attack ranged from 13 to 32 years old and included six women and girls.
In 2009, gunmen attacked several rehab centers in Ciudad Juarez, killing dozens of clients.
In other bloodshed in Ciudad Juarez, gunmen killed three undercover Mexican federal police officers as they waited for a person to cross a bridge from El Paso, Texas, authorities said Wednesday.
The Chihuahua state attorney general's office gave no further details of Tuesday's shooting, but motorists crossing the Cordova Americas International Bridge at about 1:30 p.m. were told by officials that there was a delay because of a shooting.
In an unrelated attack, a Chihuahua state police officer was killed Wednesday in his Ciudad Juarez home, said Arturo Sandoval, a spokesman for the state attorney general's office.
A territorial battle between the Juarez and Sinaloa drug cartels has torn Ciudad Juarez for nearly three years, claiming more than 6,500 lives, many of them police. The city of 1.3 million is one of the world's deadliest.
Federal police have come under increased attacks in Ciudad Juarez since taking over security in the city from the military earlier this year.
At least 115 police officers or investigators have been killed in Ciudad Juarez this year -- including 32 federal police.
A video posted online Wednesday by a local newspaper showed the kidnapped brother of a former Mexican state attorney general saying at gunpoint that the former governor of Chihuahua, Jose Reyes Baeza, had met with Juarez drug cartel leader Vicente Carillo three times at a ranch.
It was the second video released this week showing attorney Mario Gonzalez -- the brother of former state attorney general Patricia Gonzalez -- sitting in a chair, handcuffed and surrounded masked men pointing guns at him. The video was posted by the Diario de Juarez newspaper.
Reached by telephone, Patricia Gonzalez said she could not comment on the latest tape, and a spokesman for the state attorney general's office said the same.
Gonzalez stepped down Oct. 3 as Chihuahua attorney general when a new governor took office. Her brother was kidnapped Oct. 21.
In the previous tape, the kidnapped attorney said he and his sister worked for the Juarez cartel. On the tape released Wednesday, Gonzalez said: "I ask forgiveness from God and the people for having killed so many innocent people. We were sick with power, me, my sister, the governor ... and all of the people in the Juarez cartel."
It is often impossible to verify the accuracy of the admissions made under extreme pressure. It is a tactic that has become increasingly common in Mexico's brutal drug war: cartels kidnapping police, officials and regular citizens and releasing video clips of the captives admitting to crimes.
In the northwest city of Culiacan, meanwhile, gunmen burst into a Red Cross hospital and kidnapped a young man who had been shot, said Martin Gastelum, a spokesman for the Sinaloa state attorney general's office.
The 23-year-old man had just been brought in with a gunshot wound when the armed men burst in, opened fire and hauled him out, Gastelum said.
Gastelum said he did not have further information but did not believe any Red Cross workers were hurt.