Washington ? Tom Ridge, the nation’s first homeland security secretary, announced Tuesday that he was 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 oversaw 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 would 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 had foiled any terrorist attacks inside the United States, but he said the country was 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.

“His efforts have resulted in safer skies, increased border and port security and enhanced measures to safeguard our critical infrastructure and the American public,” Bush said in a prepared statement Tuesday evening.

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

“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 exits are expected, and administration officials say Health and Human Services Secretary Tommy Thompson appears to be next.

Secretary of Homeland Security Tom Ridge, center, thanks members of his staff and employees of the Office of Homeland Security as he returns to his office after his press conference announcing his resignation in Washington. Ridge whose name became synonymous with color-coded terror alerts and tutorials to the public about how to prepare for possible attack, submitted his resignation in writing to President Bush on Tuesday.

The turnover in the Bush Cabinet is typical of second-term presidencies. Presidents Clinton and Reagan had seven Cabinet seats change hands after they won new terms, President Nixon nine and Presidents Truman and Johnson four each.

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.