WASHINGTON — The world's largest defense contractor has agreed to pay $4.7 million to settle charges that it illegally used government money to lobby top federal officials to extend its contract to run one of the country's premier nuclear weapons labs.

Over five years starting in 2009, top executives for Lockheed Martin — who were being paid by the federal government to run Sandia National Laboratories — ran a fierce campaign to lobby members of Congress and senior Obama administration officials for a seven-year extension of their contract, according to the settlement the Justice Department announced Friday.

Lockheed executives, who hired a former New Mexico congresswoman to help them, didn't just press people with influence to re-hire them for a deal worth $2.4 billion a year, as Energy Department Inspector General Gregory Friedman disclosed in an investigation last fall. They urged them to close the bidding to competition.

"The money allocated by Congress for the Sandia National Laboratories is designed to fund the important mission carried out by our national laboratories, not to lobby Congress for more funding," Principal Deputy Assistant Attorney General Benjamin Mizer, head of the Justice Department's Civil Division, said in a statement.

Mizer said the lab, a subsidiary of Lockheed, allegedly used federal money to lobby Congress and other federal officials from 2008 and 2012, "to receive a non-competitive extension" of its contract in violation of federal law.

The case opened a window on the inner workings of power and influence in Washington. It's not surprising that a big, politically connected defense contractor would lobby hard to keep a lucrative slice of federal business. But this case went further. It was taxpayers, not Lockheed's corporate lobbying arm in Bethesda, Maryland, who paid for the influence peddling.

"Using public funds to lobby for a non-competitive extension of a contract is simply unacceptable," Friedman said in a statement.

Heather Clark, spokeswoman for Sandia National Laboratories, said the lab "has agreed to settle with the Department of Justice to put the matter behind us, take action on what we learned and focus on our important national security mission."

She said Sandia executives "believed our actions for a contract extension fell within allowable cost guidelines," but now realize that they "acted too early and too independently in planning for a possible contract extension."

The investigation has "clarified" Sandia's understanding of "our legal obligations on interacting with public officials," Clark said.

To clinch the contract extension, Sandia labs officials hired high-priced consultants — including a former congresswoman who was paid $226,000 — to write up a "contract extension strategy." Among the tactics suggested by Heather Wilson, R-N.M. were "working key influencers" by targeting then-Energy Secretary Steven Chu's staff, and even his family, friends and former colleagues at another federal lab, all with the goal of keeping Lockheed Martin in charge of Sandia in Albuquerque.

In an investigation last fall, Friedman revealed how Sandia hired Wilson's consulting firm and two unnamed former employees of Energy's National Nuclear Security Administration, which oversees the nuclear labs. Wilson's company, Heather Wilson LLC, gave explicit guidance to the Sandia team on how how to influence the most important people in Washington who would decide whether Lockheed's contract would be renewed.

"Lockheed Martin should aggressively lobby Congress, but keep a low profile," she advised.