By Amy Sawitta Lefevre
BANGKOK, March 31 (Reuters) - A Thai criminal court on Monday dismissed a case against five men, including a senior police officer, charged with murdering a Saudi businessman over precious stones worth millions of dollars stolen a quarter of a century ago.
The theft of the jewels worth $20 million in 1989 and subsequent disappearance of Mohammad al-Ruwaili, a businessman with ties to the Saudi royal family, strained Thai-Saudi relations severely.
Bangkok's Ratchadaphisek Criminal Court ruled there was insufficient evidence to try the five men who were accused of kidnapping and killing Ruwaili, who had gone to Bangkok to investigate what became known as the "Blue Diamond Affair".
The gems and jewellery were stolen from a Saudi prince's palace by a Thai gardener, Kriangkrai Techamong, who shipped the loot back home to Thailand.
Kriangkrai was arrested soon after the theft but he had already sold many of the stolen gems, including a priceless 50-carat blue diamond. Some of the jewels were eventually returned to their owner but Saudi Arabia later complained most of the returned gems were fake.
Saudi authorities, distrustful of the Thai police investigation, sent Ruwaili to Bangkok to investigate the case.
Three Saudi diplomats were shot execution-style in Bangkok days before Ruwaili vanished in 1990, and Saudi Arabia has long suspected official involvement in the killings.
Thailand's Department of Special Investigation, the Thai equivalent of the U.S. FBI, has denied that the murders and disappearances are related to the 1989 jewellery heist.
The abduction and disappearance of Ruwaili and the murders of the three diplomats remain unsolved, and Monday's court verdict will likely strain Saudi-Thai relations further. (Editing by Paul Tait)