The tribute was unveiled in December amid reports that Taiwan and the mainland discussed exchanging imprisoned spies, although Taiwan denied the reports. Last week, officials from both governments engaged in historic talks - the first since the bitter split in 1949. Beijing regards Taiwan as part of one China.

The monument stands as a reminder that mainland Chinese only recently learned about the events in Taiwan. Public discussion about the agents' execution was taboo on the mainland before 1990. But while more people now know the history, many still do not know what happened to their relatives who spied for the Communists. A few of them, with the help of a Taiwan-based human rights group, have travelled to Taiwan and returned with their loved ones' remains. Most graves, though marked, are situated at undisclosed or long-abandoned places.

Dai Xiaoping found out what happened to his father only after years of searching for answers.

"I had been looking for my father since my youth," Dai, 69, a retired electrical engineer in Shanghai, says. "I had no idea what had happened to him until 1964, when the authorities finally told my mother that he had been sacrificed in Taiwan in 1951."

Dai's father, who had studied at the Jiangsu Police Academy, had joined the Nationalists to stop the Japanese invasion in the 1940s. In 1945, he secretly joined the Communist Party and was sent to Taiwan with seven others. Dai says that when his mother died years ago, her last wish was for him to find the remains of his father and bring them back to the mainland for proper burial.

"But back then, relations with Taiwan were still sour, making it difficult for me to locate my father's remains, especially since I had only an old picture of him."

But he grew hopeful in 2008, when relations between the two governments warmed after Ma Ying-jeou became president. Two years ago, through the help of the Taiwan Mutual Assistance Association for Political Victims, he finally found his father's tomb in Liuzhangli, Taipei. He says he asked the mainland authorities for help to retrieve the remains, but they did nothing. He will try to do so on his own during the grave-sweeping festival in April.

History haunts those in Taiwan as well.

Taipei resident Huang Hsin-hua lived through the White Terror, when political dissidents were suppressed under the martial law imposed on the island in 1949. It was not until 1987 that Chiang's nationalist government finally lifted martial law and stopped the pursuit of communist spies. By then, Chiang's government had arrested up to 8,000 people and executed at least 1,000. Many of those killed were mainlanders who had followed his forces to Taiwan or native Taiwanese who had joined the underground Communist Party on the island.

Huang, 63, says that when she was in junior high school, her mother told her that her father had been executed by the Nationalists in 1951. "I was shocked and scared," she says, adding that she rarely told others about her father until 1995, when then-Taiwanese president Lee Teng-hui apologised for the killings.

Retired PLA general Xu Guangyu says the long-standing hostility between Taiwan and Beijing has made the search for those who died during the period difficult, especially as most were secretly executed.

"It explains the delay in building the memorial square until late last year," he notes.

The history of the four agents depicted on the memorial - Wu Shi, then deputy chief of the general staff of Taiwan's Defence Ministry; Chen Baocang, then superintendent of the 4th army corps; Nie Xi, Wu's aide de camp, and female Communist spy Zhu Feng - was not well known to the mainland public for decades. But since the late 1990s, the public has learned bit by bit about their work and fate.

The 1993 discovery in Liuzhangli of 200 tombs containing the remains of many of those executed prompted field studies by historians and human rights workers, whose work contributed to the disclosure of information. In 1995, retired Taiwanese intelligence chief Ku Cheng-wen published his memoirs about the killings. And in 2003, Taiwan's military declassified confidential information about the mass hunting of Communist spies during the 1950s.

Many mainlanders learned the historical details in 2000, when Taiwanese editor Francisco Hsu published photographs and illustrations, including the moments Wu, Zhu and two of Wu's aides were court-martialled and sent for execution.

Wu, a Kuomintang, or Nationalist, lieutenant general from Fujian, secretly joined the Communist Party in the late 1940s. The party intelligence bureau sent him to gather details about KMT troops' movements after their retreat to Taiwan in August 1949.

After Chiang declared martial law on the island, Wu struggled to pass information to the Communists. The intelligence bureau sent female spy Zhu to Taiwan later that year. She relayed information after seven meetings with Wu, before Taiwan captured a leader of the underground party that led to the agents' arrests.

It was a rainy afternoon on June 10, 1950, when a Taiwanese military court sentenced the four spies to death for violating the anti-sedition law, according to a June 11 report by the in Taipei printed in Hsu's publication. The daily reported that the four were taken to Machangding, a major execution site, and made to kneel in a row facing a firing squad.

The photos sparked wide discussion about how the mainland authorities should deal with the executed agents. One picture, showing Zhu leaning against a courtroom railing after receiving her death sentence, caught the attention of her family in Nanjing. It prompted them to embark on a decade-long pursuit to obtain her ashes, which had been misplaced, for burial on the mainland. Their subsequent success in retrieving her remains in 2010 was widely reported by the media in both the mainland and Taiwan.

Xu Bodong, director of the Beijing Union University's Taiwan Institute, says that with so few survivors left in Taiwan and political hostility easing since 2008, the mainland authorities must speed up efforts to salvage and publish all information related to the historic event.

"Back when all Communist Party members on the mainland were celebrating the founding of the Chinese People's Republic, the underground members in Taiwan were either executed or locked behind bars," Taiwanese editor Hsu says.

"Many died, and some who were lucky escaped to the mountains to hide ... Rarely did any survive, and those who lived are already way past 90. The authorities must hurry to publish a book in memory of the underground members."