“No crew was found on board the cargo vessel,” a law enforcement source said on Wednesday.

“We suspect it was used in a cross-border smuggling operation because more than 1,000 boxes of frozen meat were found on board.”

As the bow was damaged, “it was likely the two boats collided and the crew from the cargo vessel was picked up by another boat, which fled before rescuers arrived”, he said.

The source said no registration number was found on the cargo vessel. “This type of boat is rare in Hong Kong. We don’t rule out the possibility that it came from the mainland,” he said.

Police had been tasked with investigating the cause of the incident, he said.

A police spokeswoman said the vessel was found in the vicinity and suspected to be linked to the incident. It was now moored at the marine police base in Tuen Mun. The marine police regional crime unit was handling the case.

No arrests have been made.

The fatalities happened shortly before 9.15pm on Tuesday when the customs craft was on patrol off Sha Chau.

There were five officers on board the customs launch.

The three killed were senior customs officer and mother-of-three Ng Wing-man, 43, acting senior customs officer Lai Chi-hang, 39, and customs officer Wong Cheuk-bond, 26.

For the rescue operation, the Fire Services Department deployed five fireboats, four fire engines and nine ambulances.

The first fireboat arrived at the scene 29 minutes after the crash.

Divers found the three victims unconscious in the launch’s wheelhouse, which had a broken window.

The three, who had been wearing life jackets, were taken to Tuen Mun Hospital, where they were later certified dead.

The two survivors, aged 27 and 30, suffered minor injuries when they fell overboard and were treated at North Lantau Hospital in Tung Chung before being discharged.

Chief Executive Carrie Lam Cheng Yuet-ngor, who is visiting Davos in Switzerland for an annual economic forum, issued a statement expressing her shock and sadness.

“I am profoundly grieved at the unfortunate loss of three customs colleagues who were on duty last night and I offer my deepest condolences to their families,” she said.

The central government’s liaison office in the city said in a statement that its director, Luo Huining, was saddened by the deaths and expressed deepest condolences to the families. The Hong Kong and Macau Affairs Office under the State Council also expressed its condolences.

Chief Secretary Matthew Cheung Kin-chung visited relatives of the victims at the hospital, along with Secretary for Security John Lee Ka-chiu and Commissioner of Customs and Excise Hermes Tang Yi-hoi.

“We have contacted the three families and I promise we will make every effort to support them,” Cheung said, as he praised the victims’ bravery and dedication to their jobs.

Hong Kong authorities have stepped up patrols in Hong Kong’s western waters near Sha Chau after noticing a rising trend of cross-border smuggling activities.

On January 5 and 6, customs officers intercepted three mainland-registered vessels – two fishing boats and one for cargo – off Sha Chau and seized 20 tonnes of frozen meat worth HK$1.1 million. Nine mainlanders were arrested.

On Friday, customs officers foiled another cross-border smuggling case with the arrest of 30 men and seizure of 146 tonnes of frozen meat worth HK$5.1 million after intercepting eight vessels including fishing boats and barges off Lung Kwu Chau.

The ill-fated customs patrol boat was towed to a Fire Services Department pier at the airport on Wednesday afternoon for inspection.

Built on the mainland, the HK$4 million (US$512,820) launch was one of a pair bought to replace two 17-year-old shallow-water craft in 2016. The two boats are made of aluminium alloy with a maximum speed of up to 50 knots.