The U.S. government has completed 500 miles of fencing along the border with Mexico, which is just 170 miles short of its goal.

At this pace, the administration expects to have at least 600 miles complete by Jan. 20 when the Obama administration takes office, Homeland Security Secretary Michael Chertoff said earlier this month.

Last week, President-elect Barack Obama said he wants to evaluate what is working on the border as he considers whether to continue building the fence. But by Chertoff's estimation, there will not be much left to build.

Homeland Security officials earlier this year said the fence would not be completed by December 31, as planned. About 160 miles have been built since August, despite some groups' calls for a moratorium on construction. As of December 12, 500 miles had been completed, Customs and Border Protection spokesman Mike Friel said.

The fence along the U.S.-Mexico border is not intended to stop illegal immigration altogether, but make it more difficult for people to get into the U.S. illegally, Bush administration officials say. It has been controversial and has faced several lawsuits, none of which have been successful so far. In 2005, Congress authorized the fence to help secure the border and slow illegal immigration, and then gave the homeland security secretary the power to waive the federal laws. Obama voted for the fence.

The Sept. 11 attacks revived the immigration debate and advanced the idea of a border fence. Intelligence officials have said gaps along the southwestern border could provide opportunities for terrorists to enter the United States.

The overall plan for security on that border includes additional Border Patrol agents, more enforcement of immigration laws, the fence and a high-tech "virtual fence" with surveillance technology. The administration met its goal of adding 6,000 new agents to the Border Patrol force by the end of this year, bringing the total to about 18,000.

Boeing Co. has the contract for the technology portion of the fence, as well as for some construction work. Boeing's contract for the technology is up in 2009. If the administration is not satisfied with the work Boeing has done, the Government Accountability Office — Congress' investigative arm — suggested that a new contractor take over. This would be a decision for the Obama administration. As of September, GAO said Boeing had $933 million worth of work on the project.

Since 2006, Congress has set aside $2.7 billion for the fence. But there is no estimate how much the entire system — the physical fence and technology — will cost to build, let alone maintain. In September, Customs and Border Protection Commissioner Ralph Basham told Congress that his agency needed an additional $400 million to complete the project, citing higher costs for fuel, steel and labor. Congress approved the $400 million and the Bush administration believes it now has enough money to finish the fence.