Increased pressure on drug trafficking organizations and their traditional smuggling routes into the United States has led these groups to increase the construction and use of underground tunnels to bring narcotics into the country, U.S. officials said this week.

Moreover, drug cartels are willing to invest more money and technology into constructing more sophisticated tunnels that are outpacing efforts to detect them, the officials told a Senate panel.

“For example, we have information that the cartels are purchasing highly sophisticated equipment that is capable of cutting through metal and concrete at costs as high as $50,000 to $75,000,” Laura Duffy, the U.S. Attorney for the Southern District of California, told the Senate Caucus on International Narcotics Control on Wednesday. “In 2008 and 2009, three incomplete tunnels were discovered that were created with horizontal directional drilling equipment, which constructs a path without disturbing the top soil.”

One tunnel detected last November that was 90-feet below ground and stretched a half-mile from a kitchen in a Tijuana, Mexico, residence to a warehouse near the Otay Mesa, Calif., port of entry included a rail system, electricity and ventilation, is estimated to have cost more than $1 million and take longer than a year to build, James Dinkens, executive associate director for Homeland Security Investigations at Immigration and Customs Enforcement, told the panel in his prepared remarks.

Duffy said that efforts to counter the advanced tunneling capabilities have met with mixed results, pointing to the need to “identify or develop technologies that can effectively detect the early phases of tunnel construction.”

For now, the best way to detect tunnels is by law enforcement means, usually through human intelligence, Duffy said.

Since the first cross border tunnel was discovered in 1990, 154 more tunnels have been found, all but one located in the southwest border region, the officials said. And since 2001, 125 completed tunnels have been discovered, Dianne Feinstein (D-Calif.), co-chair of the Senate Caucus, said in her opening statement. The majority of the tunnels found have been along California’s border with Mexico.

While many of the tunnels that have been discovered are labeled sophisticated, most are less so and include making use of existing subterranean infrastructure such as storm drains or are shallow and relatively undeveloped, Duffy said.

Despite the lack of technology to detect tunnels, law enforcement efforts are paying off.

Dinkens said that nearly half of all the tunnels that have been detected were not operational yet, a trend that has been increasing. This demonstrates the success of an interagency task force that is combating the use of tunnels by drug cartels as well as increased cooperation with Mexico, he said.

While the border tunnels are mostly used to smuggled drugs into the United States, Feinstein said they are probably also used to transport people and weapons and could be used to transport a terrorist into the country.

To help further combat border tunnel activity, Feinstein this week introduced legislation to make the use, construction or financing of a border tunnel a conspiracy offense and allow for wiretaps even if drugs or other contraband have not been encountered.