Former deputy national police chief Pol Gen Salang Bunnag died on Sunday after falling from the 7th floor of a shopping centre in Nonthaburi. He was 81.

His son, Pol Lt Col Hemmachak Bunnag, said Sunday that his father had suffered from depression for several years, a condition which may have contributed to his death.

Police have not officially concluded whether he had committed suicide or suffered a fatal accident. But a video clip released online appeared to show him intentionally letting himself fall, and he left behind a lengthy note to family and the public.

The clip shows a man walking alone inside a shopping mall. He approaches a glass barrier and climbs over it before falling.

Pak Kret police were informed of a man plunging from the seventh floor at a shopping centre on Chaengwattana Road at about 11am. This was later confirmed to be Pol Gen Salang.

Pol Capt Thanawat Cheewitsophon, an officer at Pak Kret police station, said that police found several handwritten notes signed by Pol Gen Salang near the body.

The letter said he had less than two years to live and wanted to offer society some benefit when he died.

He urged the public to oppose plans to build a double-track railway line with a track width of only one metre and elevated trains, and instead requested that public push for the construction of "autobahn" express highways.

Police deputy spokesman Pol Col Kritsana Pattanacharoen said national police chief Chakthip Chaijinda offered deep condolences to Pol Gen Salang's family.

"He did many good things for the Royal Thai Police and his death is a great loss."

That is not a universal view.

Pol Gen Salang, a self-styled "tough cop" had a history of dramatic and controversial actions:

• Probably the most notorious incident in Salang's career came on Oct 6, 1976. As a police lieutenant-colonel, he led the anti-riot police to attack and kill students at Thammasat University, along with paramilitary forces and so-called Village Scouts. The bloody government attack on Thai citizens ended the three-year democratic revolution of Oct 14, 1973.

• In 1996, after police arrested "most wanted" narcotics kingpin Joe Danchang and five other drug suspects in Suphan Buri, Salang took all six suspects behind a house, out of the view of the press. Shots were fired. Gen Salang then emerged to announce that the six men had broken free from their handcuffs and tried to grab hidden guns, with the result that all the suspects were killed and all the police were unharmed. The Suphan Buri district court accepted Salang's claim.

• A strong supporter of Thaksin Shinawatra from the mobile phone tycoon's political rise in the late 1990s, Salang remained a red shirt supporter until the end. He was interrogated, but never charged, after red shirt violence at Songkran, 2010, in Bangkok, and was called in during other investigations into the financing of the red shirts.

• In October, 2008, Salang claimed he would mount an independent effort to confront anti-government yellow shirt supporters of the People's Alliance for Democracy (PAD) and "retake Government House" as they protested against then-prime minister Somchai Wongsawat.

Salang's alleged plan was to rally a group of other ex-policemen under his command, in order to besiege Government House and cut all food and water supplies to the protesters. Although Salang received much front-page publicity, his plan never got beyond the self-promotion and tough talk.

• More bizarrely, Salang was a central figure in the promotion of a quack "cure" for HIV in 2000-2002. Sold or given as a marketing ploy to Aids and HIV victims, the V-1 Immunitor "medicine" was dismissed by health authorities as ineffective, and the medicine was banned for promotion and sale.

In recent years, an increasingly troubled Salang became a campaigner for railways with 1.435-metre standard gauge track.

In his apparent suicide note, intended for his friends, children and grandchildren, he begged them to make his letter public as much as possible.

He apologised to his supporters and asked them to be proud of him for resolving to draw public attention to what he described as a media cover-up on the highway-railway subject.