“Suspects should not think they can escape investigations by crossing the border,” he said.

The three Hong Kong suspects, who were arrested by Shenzhen police between July 17 and 19 in the Longgang and Futian districts, included a 41-year-old renovation worker with a triad background.

Two mainlanders, a male and female, from Hainan and Hunan provinces respectively, were also arrested, with Guangdong police believing the five suspects had acted together in handling the stolen goods.

The brazen jewellery robbery happened on the morning of July 1, where three men wearing masks and armed with a knife, hammer and retractable baton snatched up 33 luxury watches and six bracelets worth HK$23.5 million.

Police search for wounded driver in HK$23 million Tsim Sha Tsui jewellery robbery

The trio escaped in a seven-seater car, driven by a fourth suspect. A male employee suffered facial injuries after being beaten with the baton.

Shenzhen police set up a special team on the case to investigate the case after being notified by the Hong Kong officers that the suspects had probably crossed over to the mainland.

Between July 5 and 10, police in Hong Kong arrested four suspects – three men and a woman – in connection with the robbery. Among them was a 17-year-old boy who was believed to have threatened shop staff with the baton.

Superintendent Wong Chi-wai from Kowloon West Regional Crime Headquarters said police believed that the HK$1.74 million in cash seized from the three Guangdong suspects came from selling the stolen goods.

Local authorities suspect the items were moved bit by bit to the mainland through Macau, Zhuhai and Shenzhen, where they were received by accomplices across the border.

On July 19, Hong Kong police arrested one man and three women in Tuen Mun and Sha Tin who were believed to be behind the smuggling operation.

Twenty-one watches and six bracelets missing from the robbery are still not accounted for, according to authorities.

Wong said officers were still hunting for two men – a mainlander who fled across the border, and a South Asian who has left the city by air.

Li Chi-hang, director of crime and security for Hong Kong police, said the force would continue its close collaboration with their mainland counterparts.

Guangdong police handed back the three suspects under an administrative arrangement because the city and mainland have so far failed to reach a formal fugitive transfer agreement.

The lack of such a deal means the two other mainland suspects also arrested by Guangdong police will not be transferred.

In general, mainland authorities have handed over eight suspects to Hong Kong so far this year, but the city has never transferred a fugitive across the border.

Professor Song Xiaozhuang, of Shenzhen University’s Centre for Basic Laws of Hong Kong and Macau, said the case-by-case transfer arrangement was not ideal.

Hong Kong’s Tsim Sha Tsui – the shoppers’ and robbers’ paradise in Hong Kong

Song said he hoped a mechanism for handing over criminals from both sides could be established, although he admitted there were challenges, such as defining the origin of crimes and criminals.

He said all criminal cases should be covered by the mechanism including those that involved national security.

Democratic Party lawmaker James To Kun-sun was not optimistic about a solution, because it had been a problem for more than 20 years.

He said the failure to set up the mechanism centred on Beijing’s disagreement over accepting the two principles of “non-extradition for political offences” and “non-extradition for the death penalty”, stances that were widely recognised internationally.

“[The principles] are not in the interest of Beijing,” To said. “That’s why the mechanism can’t be set up.”

The lawmaker said he believed that if the city were to compromise on the issue, it would “damage the international image and the values of Hong Kong”, especially since it already had extradition agreements with other countries that operate under such principles.