SANTA FE, N.M. — Stewart Udall, who sowed the seeds of the modern environmental movement as secretary of the interior during the 1960s and later became a crusader for victims of radiation exposure from the government’s Cold War nuclear programs, died Saturday. He was 90.

A statement from Udall’s family, released through the office of his son, Sen. Tom Udall, D-N.M., said he died of natural causes at his home in Santa Fe.

Udall, brother of the late 15-term congressman Morris Udall, served six years in Congress as a Democrat from Arizona, and then headed the Interior Department for eight years under Presidents Kennedy and Johnson.

Under Stewart Udall’s leadership from 1961 through 1968, the Interior Department aggressively promoted an expansion of public lands and helped win enactment of major environmental laws, including ones to protect endangered species.

Udall helped write several of the most far-reaching pieces of legislation, including the Wilderness Act of 1964.

More than 60 additions were made to the National Park system during the Udall years, including Canyonlands National Park in Utah, North Cascades National Park in Washington, Redwood National Park in California and the Appalachian National Scenic Trail stretching from Georgia to Maine.