It was all caught on video: Five heavily armed policemen barge into a hotel in western Mexico before dawn and march out with three handcuffed men in underwear.

But police weren't making an arrest; they were apparently taking orders from a drug gang. Just hours after the three were seized, they were found asphyxiated and beaten to death.

Mexicans have become inured to lurid tales of police collaboration with narcotics gangs during 5½ years of a drug war that has cost more than 47,500 lives. But seldom do they actually see it occur, and the video broadcast on national television was a shocker.

"One assumes that in some cities ... the municipal police work for the drug cartels," said Jorge Chabat, an expert on security and drug trafficking at the Center for Economic Research and Teaching.

"But what is different here is that there is a video. It's not the same thing to imagine that this going on, and to see it."

Police officers are shown on the video entering a hotel before escorting out three men in their underwear with their hands tied behind their backs in Lagos de Moreno, Mexico. (Jalisco state prosecutors' office/Associated Press)

Police are investigating whether the gunmen who order the police throughout the tape belong to the New Generation cartel based in western Jalisco state where the kidnapping occurred. The gang is aligned with powerful fugitive drug lord Joaquin (El Chapo) Guzman.

The delayed arrests of police officials came less than three weeks before national elections in which security and corruption are major issues. The municipality, Lagos de Moreno, is run by Mayor Jose Brizuela, a politician for the Institutional Revolutionary Party (PRI), who is running for state office in the July 1 national elections. The PRI looks poised to regain the presidency in a race where all parties are trading accusations of corruption and collusion with organized crime.

Local police corruption is rampant in local governments across political parties. Thousands of Mexico's 460,000 officers, including entire forces at times, have been fired, detained or placed under investigation for allegedly aiding drug gangs. President Felipe Calderon's plan to vet them has moved slowly, with only eight per cent passing background checks and tests as of the end of 2011. Critics accuse Calderon of setting off a bloodbath with his strategy against gangs.

Victims left hotel in underwear

The Jan. 20 video released by prosecutors late Wednesday shows a police truck pulling up to a hotel in the city of Lagos de Moreno, quickly followed by a pickup carrying four armed men in civilian clothing. A city policeman carrying an assault rifle runs over to their truck and is given what appears to be a list. Then he and his fellow officers trot into the hotel and present the list at the reception desk, apparently asking what rooms the men are staying in.

In the next segment of the video, the victims are trotted out of the hotel in their underwear with their hands cuffed behind their backs. One is being hustled along by a man in civilian dress, who stuffs him into a patrol car. The gunmen — police are investigating whether they belong to the Jalisco New Generation drug gang — appear to be calling the shots throughout, with the police officers serving as gofers.

The police then watch and wait in front of the hotel while the men's luggage and vehicle are stolen. Finally, the police truck carrying the victims follows the gunmen as they drive away in their own pickup and the stolen vehicle.

So far, seven policemen and officials of the municipal police force of Lagos de Moreno have been detained pending charges. And state Attorney General Tomas Coronado said the four men in civilian clothing also have been detained separately in other cases. He declined to say what gang they might belong to.

While the kidnapping and murder occurred in January, and the faces of several officers were clearly seen on the videos, the officers were not detained until June 6, when soldiers and state police raided a local police station. They still have not been formally charged with any crime.

"It took time to obtain the video tapes, to do the investigation, and to get the arrest warrants," Jalisco state prosecutor's spokesman Lino Gonzalez said Thursday. "We didn't have the information."

Victims had been briefly detained by police

There are still mysteries surrounding the case, including whether the gunmen thought the victims were members of a rival drug cartel.

The victims were from the northern state of Coahuila, where the violent Zetas cartel has been battling the Sinaloa cartel, allies of Jalisco Nueva Generacion.

Gonzalez said the victims, before checking into the hotel, had been briefly detained by police at the local jail for a minor infraction. They paid a fine and were released. But while in custody, "They said something indiscreet," Gonzalez said.

"Apparently they said something like, 'We're from Coahuila, and we're part of the mafia."'

It's not unusual in Mexico for detainees to boast about their connections, hoping to press corrupt police to release them.

This time, however, it backfired.

"Apparently, somebody at the jail heard the comment, and reported it to the real criminals," Gonzalez said.

Jalisco state Attorney General Tomas Coronado told local media the men had claimed to be Zetas.

He said it has never been proved the men were gang members. They may have just been in Lagos de Moreno collecting the rent on a ranch, and they are being treated simply as victims.

Chabat noted that corruption has reached so deep that in 2010 in the northern state of Nuevo Leon, seven local police officers in the town of Santiago were arrested on allegations they were working for the Zetas drug gang and that they kidnapped and killed the town's mayor, Edelmiro Cavazos, in retaliation for his attempts to stop corruption.

"There are police officers who kill the mayors they are supposed to protect," Chabat said. But this week's video "is cause for despair," he said. "It gives rise to the feeling that this is not going to be solved in the short term."