Late last week, Horner learned that he was under investigation before the news broke on Monday via Dutch publication De Telegraaf.
To carry out the investigation, the F1 team's parent company has hired an external lawyer to look into the matter.
On Friday, the investigation began and media reports state that Horner was questioned for several hours.
It is believed that talks began in the morning and only finished later in the evening. However, it seems likely that a conclusion will soon be reached after this long session.
No quick decision on Horner
Speculation about the allegations against Horner continue to circulate - rumours vary widely, but for now there seems to be no clarity from Red Bull about Horner's fate and what exactly is alleged to have happened.
Sky Sports reports that Red Bull will not make any decisions for the time being.
It is being increasingly assumed that Horner will still be at the helm of the Red Bull team when it launches its new RB20 on February 15 and during pre-season testing in Bahrain a week later.
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It also seems clear that the investigation is still ongoing, as Red Bull also revealed in a statement earlier this week.
The parent company in Austria wants to conduct the investigation carefully and not make any rash decisions. It signals a logical approach by Red Bull, who wish to proceed extremely carefully in a sensitive matter such as this.
However, it promises to be a challenging period for Red Bull if this investigation continues to drag on.
If the wraps are taken off the RB20 next week and if the case also spreads into the test days and first race weekend, it will continue to be a topic of conversation circulating in the media.
Such a scenario would be uncomfortable for all parties involved and could spread to having on-track consequences for the reigning World Champions.
In a listed company, Horner would most likely have been suspended during an investigation, but Red Bull is under no obligation to do so.
The Austrian energy drink manufacturer is not publicly traded and therefore does not have to adhere to that kind of protocol.