The BBC has seen the email which led to the suspension of the top official in the Department of Regional Development.
Paul Priestly was suspended from Minister Conor Murphy's department on Tuesday.
The move came amid controversy regarding the sacking of four non executive directors at NI Water.
The email sent by Mr Priestly on 2 July followed a meeting of the Public Accounts Committee.
At the meeting questions were asked about the independence of a review into NIW, which led to the sackings.
The email confirms that Mr Priestly drafted a letter which was later sent to the PAC by one of the review team, Peter Dixon.
Mr Dixon only made two changes to Mr Priestly's draft. Mr Dixon added a threat of legal action and a line that said that the integrity of the review team had been unfairly called into question.
At that time, the SDLP's John Dallat, a member of the PAC, said that the sentiments in the letter were "unacceptable and deeply worrying" as they "sought to to suppress" the activities of the PAC.
Mr Dixon later withdrew the letter having accepted that the PAC was simply doing its job by asking questions.
On Friday, Mr Dallat described Mr Priestly's action in drafting the letter as "most inappropriate" and "rather foolish" given his role as a senior civil servant.
BBC NI political correspondent Martina Purdy said that the PAC is an independent watchdog on government spending and activity, and that any perceived attempts to interfer with it would be "a very serious matter for a civil servant".
"However, I would stress that this is still at the investigation stage. Mr Priestly's suspension is to facilitate the investigation and is not a disciplinary penalty," she added.
Earlier on Friday, the NI finance minister said his Executive colleague Mr Murphy had questions to answer about his handling of the controversy at NI Water.
Sammy Wilson said Mr Murphy should clarify his own handling of the independent review.
Mr Wilson also raised concerns about whether Mr Priestly was being made into a scapegoat.
Mr Priestly had been in the job for two and a half years and was the chief advisor to the minister.
His suspension came a day after a UTV investigation which examined his role in relation to sackings of the four NIW board members.
They were sacked by the minister in March after an the independent review found they were culpable for a breakdown in procurement procedures which meant that £28.5m of contracts were awarded without being correctly tendered.
However, the UTV programme uncovered documents which purported to show that Mr Priestly, who jointly commissioned the independent review, was given an early draft of its report and asked for changes to be made to it.
Those changes are alleged to have directed criticism on procurement matters away from the DRD and onto the NIW directors.
Mr Priestly then gave evidence about the matter to Stormont's spending watchdog, the PAC, on 1 July.
At that hearing some members of the PAC raised questions about just how independent the review had been.
On Tuesday, the civil service said Mr Priestly's suspension was to facilitate the investigation and was not a disciplinary penalty.