With less than two weeks remaining before the literary sensation of the decade – the publication of Go Set a Watchman, the sister book to Harper Lee’s To Kill a Mockingbird – the manner in which the manuscript came to light has become shrouded in yet more mystery, with seemingly conflicting accounts from people directly involved in the 89-year-old author’s affairs.

New details have emerged that call into question the official story of how and when the work was discovered after being stored in a safe-deposit box for many years. According to both Tonja Carter, Lee’s lawyer, and the publisher HarperCollins, Carter stumbled upon the typewritten text when she was rummaging around a batch of old documents last August.

But the New York Times on Thursday revealed a very different series of events. According to the newspaper, Go Set a Watchman was found almost three years earlier – by a rare books expert from the Sotheby’s auction house, who was asked to review documents held in a safe-deposit box in Lee’s hometown of Monroeville, Alabama, where she lives today in a nursing home.

At a meeting in October 2011, the paper reports, the Sotheby’s expert, Justin Caldwell, came across the manuscript of a novel that he noticed as distinct from To Kill a Mockingbird, Lee’s celebrated account of the racial fault lines of a small town in the deep south told through the eyes of a child named Scout.

Go Set a Watchman is described as a “parent” book to Kill a Mockingbird – though it is set 20 years after the famous novel, in the same fictional town of Maycomb, Alabama, it was in fact written before Mockingbird, with Lee having been encouraged by her then editors to have another go.

Crucially, according to the Times account, the 2011 meeting was also attended by Lee’s attorney. Carter, the lawyer, is said to have acknowledged last week that she was present at the meeting, but insisted that she left the room “to run an errand” before the revelatory discovery and had remained unaware of what had occurred.

The revelation that Go Set a Watchman was unearthed three years before the official account of Lee’s team has suggested, at a meeting attended by the very same lawyer who has herself claimed to make the later discovery, casts the murky story surrounding the book into further obscurity.

When the plan to publish the novel was first announced in February, to the astonishment and jubilation of readers all around the world, local townspeople and friends of the author raised questions about the extent of Lee’s involvement in the decision.

Rebutting the speculation about the author’s personal intentions, and the degree to which the author was able to give informed consent after suffering a stroke several years ago, a statement was released that said Lee was “alive and kicking and happy as hell” with the upcoming book. The statement was made public by Carter.

Carter did not immediately respond to request for comment from the Guardian to her law offices in Monroeville, seeking clarification about the discrepancies in the accounts.

In a statement, the publisher said: “HarperCollins was first informed of the discovery of the manuscript of Go Set a Watchman by Tonja Carter and [literary agent] Andrew Nurnberg in 2014. We were not aware of the 2011 meeting, however we have no reason not to believe Tonja Carter’s account.”

In 2011, when the Sotheby’s meeting took place, Lee’s sister, Alice, was still acting as the prime legal supervisor to the estate of Nelle – as the author is known locally. Alice Lee died last November, and in the wake of her death the legal baton passed to Carter; three months after that, the existence and forthcoming publication of Go Set a Watchman was unveiled to the world.

The launch of the novel on 14 July will be greeted with global fanfare, with pre-orders of the book already placing it in Amazon’s bestselling lists. On the night of the launch, excerpts will be read at New York’s 92nd Street Y by Mary Badham, who played Scout alongside Gregory Peck in the legendary 1962 film version of To Kill a Mockingbird.