The FBI has released 128 pages from its file on Whitney Houston, revealing details of an apparently successful blackmail plot, as well as an investigation into an obsessed fan.

Released in response to a freedom of information request, the FBI's documents cover 11 years of threats against the singer, from 1988 to 1999. But the pages are heavily redacted – in many cases, to the point of incomprehensibility.

Sometimes the redactions are tantalising. In late 1992, an unidentified Chicago lawyer wrote to Houston's New Jersey-based production company stating that unless the singer paid $100,000 (£66,000), his client planned to "reveal certain details of [Houston's] private life … to several publications". Later the blackmail amount was boosted even higher, to $250,000 (£165,000).

According to the FBI, this was extortion. But when agents met with Houston and her father, the singer said she knew the woman who was making the threats, and that she was "a friend … [who] would never do anything to embarrass her". Officers closed the case, even though Houston's father had apparently sent the blackmailer a confidentiality agreement and an unknown sum of money.

In addition to the extortion case, officers investigated several cases of over-devoted fans. One Vermont letter-writer claimed: "I start to shake … when I think about you."

"Over the past 17 months, I have sent … 66 letters to Miss Whitney," he wrote. "I have tried to stop writing the letters and to give up twice but after a few weeks I had to start writing again … I have gotten mad at [Whitney] a few times [for not replying] … it scares me that I might come up with some crazy or stupid or really dumb idea … I might hurt someone with some crazy idea."

FBI agents eventually questioned Houston's one-sided pen-pal in 1988. They decided he was harmless. The same was true for a Dutch or Belgian correspondent who insisted he had written some of Houston's songs. The writer further claimed that he was the president of Europe and had purchased the country of Brazil for $66bn.

After selling more than 200m records worldwide, Houston drowned in a hotel bathtub in February 2012. She was 48.