WASHINGTON — Tom Ridge, named the nation’s first homeland security secretary after the Sept. 11 attacks, announced yesterday that he is resigning after three years of reworking American security and presiding over color-coded terror alerts. He’s the seventh Bush Cabinet officer leaving so far.

Ridge presided over the most significant government reorganization in 50 years. He’ll be remembered for his terror alerts and tutorials about how to prepare for possible attacks, including the controversial “disaster kits” that caused last year’s run on duct tape and plastic sheeting.

Amid warnings that the country may face increased terror risks around the holidays and the Jan. 20 presidential inauguration, Ridge said he will remain on the job through Feb. 1, unless his replacement is installed sooner.

Ridge acknowledged he could not prove the costly and complex security measures that have been put in place have foiled any terrorist attacks inside the United States, but he said the country is safer today than before the suicide hijackings on Sept. 11, 2001, killed nearly 3,000 people in New York, Washington and Pennsylvania.

“I am confident that the terrorists are aware that from the curb to the cockpit we’ve got additional security measures that didn’t exist a couple years ago,” Ridge told reporters at the department’s Washington campus, which he helped create.

Ridge sent his letter of resignation to President Bush at midday yesterday, after attending a morning White House threat briefing with CIA and FBI officials. The former Pennsylvania governor, Ridge thanked Bush for giving him the opportunity to fight back against terrorists, as did the passengers on Flight 93, who forced their hijacked plane down in a Pennsylvania field.

“There will always be more to do, but today, America is significantly stronger and safer than ever before,” Ridge wrote Bush.

Ridge is the seventh of Bush’s 15-member Cabinet to announce they won’t be part of the second term. More are expected, and administration officials say Health and Human Services Secretary Tommy Thompson appears to be next.

Among those mentioned as possible candidates to replace Ridge are Bernard Kerik, the former New York City police commissioner who helped rebuild Iraq’s police force; former Federal Emergency Management Agency Director Joe Allbaugh; Environmental Protection Agency Administrator Mike Leavitt; and White House homeland security adviser Fran Townsend.

Others are also believed to be interested in the job, including Asa Hutchinson, undersecretary for border and transportation security in the Homeland Security Department.

Ridge leaves behind a department that’s still learning to work together. Culled from 22 often disparate federal agencies, the 180,000-employee organization still faces criticism over aspects of its massive government merger, including the coordination of finances to computers systems.

Ridge, consistently a defender of the department, stood by its efforts to warn the public of possible terror threats, saying it preferred to disclose more information than some officials believed was wise.

“That’s something we take pride in,” Ridge said. “America is prepared to deal with the reality of the post-9/11 world. It’s in our best long-term interest to share more information about the threat to America rather than less.”

Ridge, who is married with two children, said that for the future he intends to “raise some family and personal matters to a higher priority,” including attending his son’s rugby games.

In an e-mail circulated to Homeland Security officials, Ridge praised the department as “an extraordinary organization that each day contributes to keeping America safe and free.”

In October 2001, Ridge became the nation’s first White House homeland security adviser, leading a massive undertaking to rethink all aspects of security within the U.S. borders in the wake of the Sept. 11 attacks.

Congress subsequently passed legislation establishing the Homeland Security Department, with Ridge taking over as the department’s first secretary in January 2003.

He has presided over six national orange alerts — second highest on the five-color scale — when the government boosted security out of concern that an attack may be coming.

Yet Ridge, a politician by nature, fought criticism leading up to the election from those who said he was using terror warnings to boost support for Bush. Ridge repeatedly said: “We don’t do politics in the Department of Homeland Security.”

Ridge, who has spent 22 years in public service, came home from Vietnam, earned a law degree and went into private practice in Pennsylvania. He ran for Congress in 1982 and was re-elected five times.

He became the Pennsylvania governor in 1995, leaving the state capital in October 2001 after the White House called.

The six other Bush Cabinet figures who are leaving are Attorney General John Ashcroft, Commerce Secretary Donald Evans, Education Secretary Rod Paige, Agriculture Secretary Ann Veneman, Secretary of State Colin Powell and Energy Secretary Spencer Abraham.

Bush has chosen national security adviser Condoleezza Rice for the State Department, White House counsel Alberto Gonzales for the Justice Department, Carlos Gutierrez for Commerce and Margaret Spellings for Education.