It is a bitterly cold night and the streets of east Tijuana are deserted as Officer Villasenor heads out on patrol.
His shotgun rests in easy reach across his knees.
It is not long before the first call comes in: reports of shots fired, and a taxi hijacked.
Within minutes we are at the scene, along with six other police cars. They begin criss-crossing the dense patchwork of streets and intersections, shining their torches into driveways and parking lots.
"This our strategy. We call it the blockade. When there is an incident we surround the area, we try to trap the suspects," Officer Villasenor says.
Tijuana's municipal police department has undergone a radical overhaul in the past three years.
Many officers used to be in the pay of drug cartels.
That was part of the reason that Mexican President Felipe Calderon deployed thousands of troops to border cities like Tijuana.
"It used to be common to have armed convoys of organised crime groups moving freely through the streets, ten vehicles together. Officers were afraid to intervene," says Officer Villasenor.
Things started to change when a tough new police chief was hired at the end of 2007.
Julian Leyzaola, a former army officer, teamed up with the military to take on the drug cartels directly.
"I started to buy more weapons and ammunition, a lot of ammunition, but I also trained my officers in urban combat," he says.
Chief Leyzaola did not have to wait long before his first confrontation. Within weeks of taking office, reports came in of an armoured truck being hijacked in the city centre.
"We gave chase and caught up with the truck on the highway. One of the hijackers stepped out. They weren't used to being stopped by the police. I told him, lay down your gun. He didn't obey so I shot him."
The dead hijacker turned out to be a policeman working part-time for organised crime.
"I knew that the police force was completely infiltrated. Officers would work an eight-hour shift for the police and then they would go to work a shift for organised crime, using their police weapons, even their own uniforms," said Mr Leyzaola.
He began an unprecedented purge of his force, arresting scores of officers suspected of working for the drug cartels. He replaced some of them with former army officers.
Gradually there were signs of change in Tijuana. The high-profile armed convoys of cartel members and spectacular shoot-outs between them in the city centre stopped.
But then disturbing reports started to emerge about the methods used by Tijuana's police chief.
Miguel Mesina is a veteran with more than 30 years' experience in the municipal police.
He was detained early last year as part of the Chief Leyzaola's purge, taken to a military base, and blindfolded.
"They tied me up and put me on the floor. They asked me to admit I was a member of organised crime. I didn't accept that. They sat me on a chair and started to give me electric shocks on my genitals," says Mr Mesina.
He says that during this time he could hear Chief Leyzaola's voice in the room.
Mr Mesina's 26-year-old daughter, Blanca, then began a high-profile campaign to free her father and other officers being detained with him. She directly accused Chief Leyzaola of torture.
At this point, she says, the pressure began to mount.
"Police cars started following me. Then finally, I was coming back from work, and a pick-up hit my car," she says.
"I was forced to park in a car park. The driver followed me and got out. He came and put a gun to my head and said if you don't stop complaining someone from your family is going to get hurt."
Mr Mesina was released without charge in August.
Blanca Mesina is currently in hiding. She says she does not feel safe enough to return to Tijuana.
Dozens of others, both police officers and civilians, have made similar claims of being detained and tortured to confess to crimes.
Earlier this year, a report from the Tijuana branch of Mexico's State Human Rights Commission alleged that Chief Leyzaola was present when five suspects were tortured in 2009, and that he personally participated in one of those cases.
The commission recommended that Chief Leyzaola be suspended, pending an investigation.
Mr Leyzaola vehemently denies the accusations.
"It's completely false. The attorney general's office has done an investigation. If there was evidence they would present charges," he says.
"So far, no-one has been able to prove scientifically that these people have been subjected to this kind of treatment."
Mr Leyzaola believes human rights groups have been manipulated.
"When the drug traffickers were not able to win the war on the streets with their narco-culture, they went and used human rights organisations. It's a defence strategy," he says.
Late last month, Tijuana's mayor named a new police chief, and gave Mr Leyzaola a new job as deputy secretary of public safety for the state of Baja California, which includes Tijuana.
Many businessmen are sorry to see him go.
Genaro de la Torre was kidnapped by cartel members six years ago with the help of corrupt policemen. He believes Mr Leyzaola has made the city centre safe again.
"If you are on the good side, he's the best. If you are a bad guy of course you're going to cry," says Mr de la Torre.
"You cannot be merciful to a murderer who is dealing drugs and poisoning youngsters."
President Calderon describes Tijuana as "a clear, concrete example that the challenge of security has a solution".
But in the poorer parts of the city, violence between the drug cartels continues. The murder rate this year in Tijuana has exceeded last year's total.
Most believe the drug trade still thrives.
"We are sitting on top of a volcano, and soon it will probably erupt in a violent way," said Victor Clark Alfaro, director of the Bi-national Centre for Human Rights in Tijuana.
He believes that despite the efforts to improve the municipal police, the system as a whole is still failing.
"The level of corruption of federal prosecutors and judges is scandalous. You see drug dealers arrested and shown on TV, but you find out later the majority were released."
Back out on patrol with Officer Villasenor, the night shift has quietened down by the early hours, and there is more time to talk.
The toll for the evening is one man dead, shot in the head as he left a bar, and one woman shot and injured.
Officer Villasenor and his partner are clear about the most significant change that Chief Leyzaola made.
"He cleaned the force," they say.