Mr. Semegnew was in Addis Ababa on Thursday to give a news conference at a time of growing public unease over delays in the project and concerns about corruption and mismanagement, officials said. He died hours before he was scheduled to speak.

Standing atop the dam in May, Mr. Semegnew offered his usual optimistic assessment of the project, which he said was about two-thirds completed, and he played down the fears of downstream countries, notably Egypt.

The dam had turned Ethiopia into “the power hub of the region, of the continent,” he said, and it would be used only to generate electricity, and not for irrigation.

He declined to say why the project’s completion date had been repeatedly delayed over the past year. “I cannot comment to you on this,” he said.

Those delays, and the dam’s generally opaque finances, have become matters of considerable speculation in Ethiopia in recent weeks. Some people who posted messages on social media sites suggested that Mr. Semegnew was killed because he was planning to name corrupt officials who were making money from the project or that Egypt had somehow played a role in his death.

None of the commentators offered evidence to support those conspiracy theories, but they did appear to reflect growing public disquiet about a project that is behind schedule, has consumed huge amounts of taxpayers’ money and has increased tensions with Egypt.

An upbeat visit to Cairo by Prime Minister Abiy last month improved that often-tense relationship. The visit was part of Mr. Abiy’s dizzying series of reforms, including peace overtures to rival Eritrea, since he came to power in April.