The two leading schools of thought are that it was a lone gunman, perhaps enraged by Palme’s social democratic politics, or a much more intricate plot involving the South African apartheid regime.

South African intelligence officials met Swedish investigators in Pretoria in March and handed over a dossier of information related to the association, according to sources familiar with the meeting.

It is not clear however whether the dossier included substantive new evidence, or was simply tying up loose ends in a decades-long investigation.

The article author seems to really be favoring the South Africa lead, maybe because it would be more explosive. However:

The main rival theory is that Palme was killed by an individual acting out of ideological hatred for the Social Democrat prime minister. One suspect is Stig Engström, known as “Skandia man” because he worked for the Swedish insurance company in offices next to the scene of the murder.

Engström had weapons training, possible access to the .357 Magnum revolver suspected to have been used in the killing, and politics considerably to the right of Palme. He took his own life in 2000. In May 2018, a Swedish magazine, Filter, published a 12-year investigation which concluded that Engström was probably the killer, on the grounds he matched the description of the gunman, he had information that only the killer could have known, and lied to the police about his movements on the evening of the murder.

According to Swedish newspaper Aftonbladet, the investigator* have informed the government that they've found the murder weapon, which makes me think it was the Skandia man. It's probably much easier to track down a Magnum revolver that used to belong to a collector than one provided by a foreign intelligence agency. I guess we'll know by Wednesday.

For more backstory:

* The lead investigator is named Krister Petersson, while the only person convicted but later cleared of all charges for the murder was named Christer Pettersson.