The Bush administration needs an extra $400 million to complete its fence along the country's southwestern border, and government investigators say that may not even be enough to finish construction by the end of this year.

To complete the 670-mile fence — already half built — the administration has asked Congress to approve the use of $400 million set aside for other programs, mostly surveillance technology projects along the U.S.-Mexico border, Jayson Ahern, the deputy commissioner of Customs and Border Protection, told The Associated Press Tuesday.

Higher costs of fuel, steel and labor have led to the $400 million shortfall, Ahern said.

"If we run out of money, unfortunately the construction will have to stop," Ahern said. He said it is not known exactly how much extra it will cost to build each mile of the fence, because the costs differ due to varying terrain and environmental issues.

Ahern is scheduled to testify on Capitol Hill Wednesday about the fence's funding shortfalls. At the same hearing, Congress' investigative arm, the Government Accountability Office, will also tell lawmakers that the administration risks not meeting its deadline to complete the fence by the end of the year because of staffing shortages and complications with acquiring the land necessary to build the fence.

The concept of a border fence took on new life after the Sept. 11, 2001 terrorist attacks, which revived the heated immigration debate. Intelligence officials have said the gaps along the southwestern border could provide opportunities for terrorists to enter the country.

Critics have said the fence presents an inconsistent message about a country founded by immigrants and priding itself on opportunity.

The fence is not intended to stop illegal immigration altogether, but rather make it more difficult for people to enter the country illegally, administration officials say..

The entire plan for security on the southwestern border includes additional Border Patrol agents, more enforcement of immigration laws, a physical fence and a high-tech "virtual fence" with surveillance technology.

The administration learned the high-tech portion of the fence is more difficult than anticipated after its 28-mile test of a virtual fence in Arizona. Lawmakers have hammered the administration for what they consider a failed virtual fence plan, but Ahern and other officials have said that criticism is not fair.

While there are still plans to install virtual fencing along other parts of the border, the administration thinks it's more important to use the money set aside for technology in order to complete the physical fence, Ahern said.

As for completing the fence by end of the administration, Ahern said, "We still have many challenges that remain to be overcome." Among those challenges are getting court orders to build on certain lands. "And that's out of our control," he said. He said staffing shortages, however, are not a problem.

The fencing plan affects about 480 landowners who live along or near the southwestern border. Some citizens are faced with moving out of their homes and selling their property to the government because the placement of the fence would significantly affect the value of their property. Others could accept a government payment as compensation for reduced value. As of August 26, the administration had 269 pieces of property it still needed to acquire from landowners, according to the Government Accountability Office.

Since 2006, Congress has appropriated $2.7 billion for the fence. But there's no estimate how much the entire system — the physical fence and technology — will cost to build, let alone maintain.

Border fences have been sprouting across California, Arizona, New Mexico and Texas for decades — dating to the 1940s, when the International Boundary and Water Commission, an agency that deals with border issues, built 234 miles of fence to keep out foot-and-mouth disease.

As a result, the U.S. fence is a patchwork of old and new construction and in varying states of repair; the only consistency is a uniform ugliness. Over the years surveillance cameras, ground sensors and unmanned aerial drones have been used in spots along the border. But the current building spree is the first comprehensive federal push to seal the entire stretch with either physical fencing or detection and surveillance technology.

If Congress does not approve the use of an additional $400 million to build the physical fence, Ahern said, "We will exhaust our moneys and go as far as we can."