The lead masks case: Two men found dead wearing lead masks. What happened?

They were found on a hill with no visible injuries, a notebook with cryptic instructions, and a package containing a white towel.
In 1966, two electronic technicians from Brazil were found dead on Vintém Hill. They were wearing formal suits, waterproof coats, and peculiar lead masks with no eye holes. Near the bodies, police discovered an empty water bottle, a package containing a white towel, and a notebook with cryptic instructions that read: '16:30 be at the agreed place. 18:30 swallow capsules, after effect, protect metals wait for mask signal.' Despite extensive investigation, the cause of their deaths and the purpose of the lead masks remain a mystery.
Location: Vintém Hill, Rio de Janeiro, Brazil
Year: 1966
Investigate Further