However, the medical examiner later changed the cause of death to undetermined.
That ended last November when he went missing and his semi-frozen body was later found hanging from a high tree branch in Seattle, Washington. News stories about his death and the police investigation went viral this week - two months after he was found.
Lake Stevens Police Chief John Dyer said investigators believed it was a suicide but conferred with the Federal Bureau of Investigation and had awaited laboratory results.
The father said there was no reason for the family to suspect anything was wrong. "The Lake Stevens Police Department has submitted physical items for lab review and we await their results to provide additional pieces of the picture".
Police searched the area around the Keita family's suburban home three times between November 30 and December 7, using dogs and a helicopter in futile attempts to find him, Arsalan Bukhari, executive director of the Washington State chapter of the Council on American-Islamic relations, said in a phone interview.
Ben, a Running Start student allowed to begin their college education while still in high school, had stopped going to class a few weeks before he went missing, police said. A previous dog search of the same woods weeks earlier did not discover Keita's body at the time, indicating that he was most likely missing for quite some time before his death. The family is also asking anyone with information about the matter to come forward.
Bukhari said the police, who treat such cases as potential homicide until all possibilities are exhausted, ruled Keita's death a suicide within three days, which was "limiting".
The case was closed a week ago, but Seattle police say that they are communicating with partners to ensure that a thorough investigation is made into the young man's death.
The Council of American-Islamic Relations are representing the family in their request that the FBI open an investigation, the group said at a news conference Tuesday alongside Keita's family. "If warranted, we may conduct further investigation", Dietrich said.
"We are careful not to rush to judgment", Rev. Kele Brown, of the Plymouth Congregational Church in Seattle, said.
Father Ibrahima Keita told local reporters his son was a "very young, happy young man" with "no history of depression, anxiety, any psychological break-down at all".