Geoff Payne has been in Buenos Aires since Friday, flying in from the United Kingdom two days after his son's death, and visiting the memorial to the late One Direction singer.

Argentina's National Criminal and Correctional Prosecutor's Office No. 14 announced Tuesday that Liam's father met with officials and was briefed on the investigation ... and they say Geoff told them he would be willing to tell them everything he knows about his son's life in order to help the investigation.

The release, translated from Spanish, says Geoff told officials he wants them to investigate and figure out what exactly happened to Liam ... and it states toxicological and histopathological studies complementary to Liam's autopsy report have yet to be officially completed.

As we reported ... the initial results from Liam's autopsy showed a mix of drugs known as "pink cocaine" in his system when he died. Liam fatally fell from the third-floor balcony of his CasaSur Palermo Hotel room on Wednesday, October 16.

Authorities in Argentina also say they told Geoff his son's body must remain in the country until all their reports are finished, before being released to the family.

Meanwhile, officials say their investigation includes a review of cell phone and computer data, plus photos and videos from security cameras that require more time to analyze.

Authorities also say they are still interviewing witnesses ... including hotel workers, acquaintances, technical and medical professionals, and other people linked to Liam through their work ... all in an effort to reconstruct the final hours of Liam's life.

With Geoff in Buenos Aires, officials say he's getting a police detail while he's there ... describing Liam's dad as in a "state of logical shock."