The police are advancing their investigation into the death of Ashlee Jenae -- the content creator who died on a trip to Zanzibar earlier this month. Authorities have seized her fiancé Joe McCann's passport and are questioning him ... but only as a witness, as no arrests have been made.

As you know ... Ashlee died while on vacation with Joe, who proposed to her on the trip. The vacation was planned as Ashlee's 31st birthday celebration, which turned out to be her last.

Ashlee's family appears to have been suspicious of Joe the whole time, as they say he waited 11 whole hours to tell them about the incident ... and even then downplayed it as if Ashlee was "okay."

As we reported ... Joe and Ashlee were separated by hotel staff into different rooms April 8, following a domestic dispute. Joe told authorities he found that Ashlee had "hanged herself on the door" on April 9 and she reportedly died at the hospital on April 10.

In a statement, police say the incident is linked to the couple's domestic dispute. Docs list Ashlee's "immediate cause of death" as cerebral hypoxia by strangulation and suffocation.

Notably, the family told us ... they haven't heard from Joe since he called to say Ashlee was in the hospital. They're considering flying to Tanzania to meet with officials there.