Acting on a tip, 30 masked soldiers in combat gear bust down the door of a boarded-up house to find 55 terrified migrants, hostages of the Gulf drug cartel.

Amid screams and the smell of urine and sweat, they find a blood-spattered room and a nail-encrusted log used to beat the captives and extort money from their families: $3,000 each.

Five suspected kidnappers are hauled off in a military truck, including the alleged leader — the son of a local police officer.

The Associated Press spent five days on the front line of Mexico's drug war, embedded with the army's 8th Division in Tamaulipas state, one of many organized-crime hotspots now policed by 45,000 troops nationwide. Launched by President Felipe Calderon in December 2006, the army is Mexico's last and best hope to gain control over drug cartels and spiraling violence, which have killed more than 9,000 people since then.

But the AP's exclusive front-row seat reveals the army offensive to be at once successful and imperfect, marred by police corruption, lack of training and local distrust. As Calderon has said, it's a temporary fix. There's still not a long-term solution.

Many Mexicans see the army as the only government entity able to face the heavily armed drug cartels, and soldiers rely on citizen complaints, such as the call that led them to the migrant hostages. They enter the house in a rough neighborhood without working with local police to get a search warrant, fearing officers could tip off the smugglers.

Army officials acknowledge they break rules to get results. Their fight is complicated by deep-rooted corruption among local and state police, who work as lookouts and sometimes hit men for the cartels.

"Here you can't call police," says army Capt. Huascar Santiago, "because they're in collusion."

The problem is also complicated by the constitution, which bars the army from doing police work such as the smuggling ring bust. Among other limits, soldiers legally can detain only people caught in the act of a crime as they check suspicious cars, rummage through trash cans and gather intelligence from neighbors.

Army Gen. Edgar Villegas, the division commander, says the military still maintains discipline in these situations.

"If we're going to act in this gray area, in the end what prevails is the honesty and transparency with which we do things," he says. "We're susceptible to committing errors, and when we do, we take responsibility for everything that comes with it."

Dozens Held, Some for Up to a Month

In the Reynosa raid, the soldiers free nine women held in the living room in their underwear and 46 men crammed into two small bedrooms — some for up to a month — with little food and water. The torture room has a mattress on the floor and blood and posters of half-naked women on the walls. A handgun sits on a corner table.

The soldiers handcuff the ring leader and cover his head. He is taken into the bathroom, made to kneel in a bathtub beside a bucket of water. The door is shut. The suspect emerges wet and willing to reveal the addresses of two other smuggling houses, though they yield nothing.

"You're heroes. God will reward you," reads a text message on Santiago's cell phone from the man who gave him the tip.

Drug traffickers once had free rein in Tamaulipas, which borders Texas and the Gulf Coast — the home base for the Gulf cartel. They raced around in convoys of bulletproof sport utility vehicles, setting up roadblocks to protect turf and forcing Mexican customs agents at gun point to wave through cars coming from the U.S. without inspection. Men openly displayed their weapons as they drank in bars or had their ostrich-skinned boots shined in the town plaza.

That was before Calderon took office and sent the army — mostly twentysomethings from rural provinces — to wrest control of areas taken over by cartels.

The 8th Division — 2,400 troops plus 1,500 reinforcements — was deployed in late 2007 after a former border town mayor who denounced cartel meddling in local elections was shot dead outside a restaurant.