The theft of copper wire has become so pervasive that it poses a threat to the national infrastructure, the U.S. Federal Bureau of Investigation warned Wednesday.There's a lot of copper lying around the U.S., in cellular towers, telephone lines, electrical sub-stations and even vacant homes, and ripping it up and stealing it has become a lucrative activity for organized gangs and drug addicts over the past few years, according to the FBI.Such thefts are typically prosecuted by local authorities but the FBI is now taking it more seriously, the agency said in a statement. "On the surface, it could be a relatively small theft," the FBI said, quoting an unnamed field agent. "But the public safety impact could be significant.""While copper thieves may not intend to compromise critical infrastructure, they can still be charged with more weighty federal crimes," the FBI said.In the past year, copper thefts have wrought havoc with Federal Aviation Administration towers in Ohio and with a tornado warning system in Jackson, Mississippi. And in May, about 4,000 residents of Polk County, Florida, lost power after thieves stole copper from a local transformer, the FBI said.They also kept the "Yappin with Yohe" radio talk show off the air in Ashland, Kentucky, according to a local report. During the early morning hours of Aug. 21, the thieves climbed poles and cut down 600 feet of copper phone cable, knocking local radio station WLCG offline and disrupting phone service to 900 customers.Copper was trading at just over US$1 per pound in 2005, but prices spiked in 2006 and have hit highs of more than $4 per pound in the past year.Although prices have dropped below $2 in the past few months, copper can still be sold to scrap-metal dealers for quick cash. The FBI says that with the current economic downturn, it is "likely that copper thefts will remain a lucrative financial resource for criminals."