WASHINGTON — President Obama on Monday signed into law legislation to provide health care to thousands of sick Marine veterans and their families who were exposed to contaminated water at Camp Lejeune in North Carolina.

Retired Marine Jerry Ensminger and cancer survivor Mike Partain stood looking over the president’s shoulder as he, with the swipe of his pen, vindicated all their late nights poring over undisclosed documents, cross-country trips to seek out other victims, and countless battles with Marine Corps officials who, they say, continue to ignore their pleas.

“Sadly, this act alone will not bring back those we’ve lost, including Jane Ensminger,” Obama said before signing the bill, named partly after Ensminger’s daughter, “but it will honor their memory by making a real difference for those who are still suffering.”

Janey Ensminger was just 9 years old when she died in 1985 of a rare form of leukemia. Her father spent years trying to make sense of her painful death.

But in 1997, he saw a news report about contaminated water at Camp Lejeune. Janey Ensminger was conceived at the base in the 1970s and diagnosed with leukemia in 1983.

Her father’s life then turned into a David-and-Goliath story, as he and Partain took on the 236-year-old Marine Corps, culminating with the signing of the law in the Oval Office.

“I’m still in shock,” said Partain. “We’ve been fighting for justice for so long. Fighting the juggernaut of the Marine Corps. They should have quashed us a long time ago. And they almost did.”

Despite its previous contention that there was insufficient evidence to prove the illnesses were related to service at Camp Lejeune, the Marine Corps said in a statement Monday that it was pleased and supported the new law.

Partain, who was born on the base, already had been diagnosed with male breast cancer when he learned of Ensminger’s efforts. A claims adjustor for State Farm Insurance, Partain figured his investigative skills would be helpful to their mutual cause.

Their combined efforts eventually led to the passage of a bill, introduced by Sen. Richard Burr, R-N.C., that would provide health care for people who lived or worked at the base from Jan. 1, 1957, through Dec. 31, 1987. They also must have a condition listed within the bill linked to exposure to dangerous chemicals.

The law is expected to help thousands of veterans and their families who were exposed to drinking water that was poisoned with trichloroethylene, tetrachloroethylene, benzene and vinyl chloride.

McClatchy Newspapers obtained documents in 2010 showing that potentially as much as 1.1 million gallons of fuel, containing benzene, leaked from underground tanks on the base.