MEXICO CITY (AP) — Fuel thieves in Mexico drilled 10,363 illegal taps into state-owned pipelines in 2017, or an average of about 28 every day.

The government-owned oil company Pemex reported Wednesday that illegal taps rose about 50 percent last year, from 6,873 in 2016.

Those are just the taps that have been found. Many others are believed to be still working surreptitiously, drawing off gasoline or diesel that is sold illegally by vendors or even through established gas stations.

Pemex said in a statement that authorities have closed 70 gas stations for selling presumably stolen fuel to drivers. The company said patrol and technological surveillance were increased and almost 4 million gallons (15 million liters) of stolen fuel had been recovered.

Last year, Mexico's government estimated the 2016 thefts cost Pemex about $1 billion for the year, but the company provided no monetary estimate of the losses in 2017.

Just five years ago, authorities found only about 1,635 illegal taps annually.

The thefts are carried out by drug cartels and other heavily armed criminal gangs.

In the northern border state of Tamaulipas, for example, the army reported Wednesday that soldiers found several stashes of stolen fuel, drugs and weapons this week.

Raids near the border city of Reynosa turned up about 5,500 gallons (21,000 liters) of presumably stolen fuel, cocaine, 17 assault rifles, two .50-caliber sniper rifles and two claymore mines. Three people were detained.

Fuel thefts were once largely confined to two or three states in Mexico, but have since spread across the country, including the suburbs of Mexico City.

"We face a great challenge, and all together, the public and the government, we should work in a grand alliance to face this criminal activity that affects the property of all Mexicans," Pemex said.

Thieves use hand drills to make a hole in pipelines and quickly screw valves and hoses into place. They then fill everything from 275-gallon (1,064 liter) portable tanks to entire tanker trucks.

The illegal taps are usually discovered either because they leak, cause pipeline pressure to drop or catch fire.