Donald W. Keyser developed a reputation as a brilliant and erudite expert on Asia in a career than has spanned more than three decades in the State Department.

According to his former colleagues, Keyser was a fluent speaker of Mandarin who, during his three tours at the U.S. Embassy in Beijing, returned from meetings with Chinese officials, sat down and batted out well-organized cables reporting the gist of the talks -- all without notes, the awed colleagues said. When making public speeches, he delivered extemporaneous remarks in lucid and concise prose.

Now Keyser, 61 and on the cusp of retirement, is accused of passing documents to Taiwanese intelligence agents and making a secret, unauthorized trip to the island. FBI agents who tailed Keyser to an Alexandria restaurant twice this summer said they watched him pass documents to two Taiwanese agents, according to an affidavit filed Wednesday in U.S. District Court in Alexandria.

Keyser, who has lived in Fairfax County since 1988, was released on $500,000 bond on a charge of lying about the trip to Taiwan on an official government document. The affidavit and a senior administration official said that Keyser made the unsanctioned trip after official visits to China and Japan and that he met one of the Taiwanese agents, a 33-year-old woman, in Taipei. The court documents show he spent $570 at a Christian Dior store during the trip. He later met the same female agent and her supervisor and passed the documents in the Washington region, according to an FBI affidavit.

People who worked alongside Keyser over the past several years were astonished yesterday at the news. They described a pragmatic man who was evenhanded in his dealings with China and Taiwan, despite the cross-strait tensions. One former colleague described Keyser, whose wife is a CIA officer, as the quintessential "straight arrow." Friends and colleagues noted the irony that Keyser, who never needed reference notes himself, is accused of having handed over papers captioned in the charging documents as "discussion points."

"He is intellectually without peer," said Jeff Bader, a former senior State Department and National Security Council official who worked closely with Keyser for 22 years. "He knew more about the U.S.-China-Taiwan relationship and how to navigate it than anybody I know. I never heard a syllable out of him in 22 years that suggested anything other than absolute loyalty and patriotism to the United States. I can't accept the notion that he has done anything of the sort that's implied."

The United States shifted its diplomatic recognition to Beijing from Taiwan in 1979, yet remains Taiwan's biggest ally and arms supplier. The Taiwan issue is at the center of the U.S.-China relationship. China claims sovereignty over Taiwan and has threatened to seize the island by force. Taiwan claims to be an independent country. In recent months, U.S. officials have expressed concern about rising tensions across the Taiwan Strait and the risk of U.S. forces being dragged into a conflict there.

Keyser was "extremely balanced and judicious in his understanding of the China issues," said David Shambaugh, director of the China policy program at George Washington University. "He was no sympathizer with either part of the cross-Straits standoff."

In the eyes of some diplomats, it is not necessarily alarming to know that Keyser handed the Taiwanese agents "discussion points."

"That is a common practice when you have discussions with foreign officials," said one former State Department official, adding, "I would take the talking points for use with the foreign officials, put them on non-letterhead paper and leave them behind after the meeting so that the officials had a precise account of what the U.S. position was."

Neither Keyser nor his attorney, Robert Litt, returned phone calls yesterday. No one answered the door at Keyser's Fairfax Station home. At a State Department briefing, spokesman Richard A. Boucher said that officials there had been aware of the FBI investigation for months and that they were cooperating.

He said that since stepping down in July as principal deputy assistant secretary for East Asian and Pacific affairs -- the No. 2 job in the bureau -- Keyser had been assigned to the Foreign Service Institute in Arlington. The institute is effectively a way station for retiring foreign service officers.

Keyser is a Baltimore native who graduated from the University of Maryland in 1965. He attended the Stanford Inter-University Center in Taiwan for two years and worked toward a doctorate at George Washington University.

He joined the State Department in 1972, the court documents say. Over the years, in addition to serving three times in Beijing and twice in Tokyo, he was the department's director of Chinese and Mongolian affairs. During the Clinton administration, he was named ambassador as a special negotiator for conflicts in Nagorno-Karabakh and the New Independent States that were formerly Soviet republics.

"The hardest thing to understand is that the word 'integrity' comes to mind when I think of Don Keyser," said Chas Freeman, who worked alongside Keyser at the embassy in Beijing. "There is a long history of Taiwanese espionage against the United States, that is not in doubt. What is most peculiar and hard to believe is that Don Keyser would have been recruitable."

Shambaugh said Keyser typically works long days, often 15 hours, and has little time for hobbies.

"The man is in at 6:30 every morning and does not leave until 10 at night," he said. "He will not leave the building until [Secretary of State] Colin Powell leaves the building."

Bader spoke of Keyser's ability to write trenchant, analytical cables from memory. In 1982, when Bader was a political officer at the U.S. Embassy in Beijing working under Keyser, the ruling Chinese Communist Party held a party congress. Typically, four or five political officers would have been assigned to analyze it, he said. But most were on leave.

"Don said he would cover [it] by himself," Bader recalled. "He churned out an endless series of perfectly composed analyses over the course of the party congress. In the pre-word processing era, I saw him sit down and type flawless 25-paragraph cables in perfect sentences, without notes, completely from memory. He was the awe of everyone."

Now, diplomats and academics accustomed to deciphering the arcana of foreign countries are wondering whether they missed something close to home.

"We're all just collectively stunned and asking each other what to make of this," Shambaugh said.

Staff writers Lena H. Sun, Glenn Kessler, Walter Pincus and Peter Whoriskey and researcher Bobbye Pratt contributed to this report.

Donald Keyser is described as quintessential straight arrow.