President Hassan Rouhani of Iran called on Tuesday for a special court to examine the recent downing of a passenger plane that killed 176 people, as protests flared for the fourth day in several cities over the disaster and officials’ shifting explanations for it.

But even as the president made the announcement, the Islamic Revolutionary Guards Corps, a powerful arm of Iran’s military, said it had arrested a person it identified as having recorded a video of a missile attacking the plane, which undercut the military’s initial denials that Iran was responsible. The arrest was announced by Iranian media outlets affiliated with the Guards.

The contradictory messages from the president, who is elected, and the Guards, who answer to Iran’s clerical leaders, reflect the competing power centers in the Iranian government.

For three days after the crash of a Ukraine International Airlines jetliner, the military blamed mechanical failure. It eventually acknowledged that it had mistakenly shot the jet down with a missile, but insisted that “human error” was to blame, not any systemic problem.