At least 40 suspects are in custody in connection with the July 7 assassination of Haitian President Jovenel Moïse, but none appears to have been the mastermind or paymaster behind the crime, which left a dangerous power vacuum in a country already beset by crime and poverty. Now a glimmer of daylight may be illuminating a reason for that abject failure of justice — the possible criminal involvement of Haiti’s own prime minister.

This week, that man, Ariel Henry, fired a top prosecutor, who sought first to question him in the assassination and then to charge him criminally and bar him from leaving the country. Outrageously, Mr. Henry did so with no explanation to the judicial authorities or to his countrymen, beyond asserting that the prosecutor had committed “grave administrative errors.”

In fact, the grave error is Mr. Henry’s own — for stonewalling and then obstructing an investigation that has further destabilized Haiti following not only the murder of the head of state but also, in quick succession, a devastating earthquake and tropical storm. If, as was alleged by the now-replaced prosecutor, Bed-Ford Claude, Mr. Henry spoke twice by telephone soon after the killing to one of the chief suspects at the scene — a former government official who is now on the lam — he must say why. To continue refusing only deepens suspicions that the prime minister had a hand in the slaying.

Haiti, a country of 11 million people, is in a parlous state. As president, Moïse heedless of democratic niceties and intent on centralizing his own power, all but abolished elections, allowing most lawmakers’ and local officials’ terms to expire. The result is that no more than a handful of current officials came by their positions through elections.

That absence of legitimacy, along with any known motive for the assassination, has fed wide speculation that a shadowy power struggle among Haiti’s moneyed elites is underway. Notwithstanding the nation’s desperate poverty, Haiti’s powerful business interests, often wealthy families enriched by imports, utilities, natural resources or foreign aid, have long pulled strings behind the scenes.

The United States, which occupied Haiti for nearly two decades a century ago, exercises outsize influence there. An inquiry from the State Department could provide the needed incentive for Mr. Henry to provide a public explanation for his alleged 4 a.m. phone calls on the night of the assassination, about three hours after Moïse was shot to death, to the fugitive former official, Joseph Felix Badio. Geolocation data showed Mr. Badio was at Moïse’s home, where the murder was committed, when the calls were placed. His whereabouts are now unknown.

The Biden administration, along with other key international powers, backed Mr. Henry’s ascent to the prime ministership when it was contested in the days following the assassination. A 71-year-old neurosurgeon and former government official, he was seen as an elder statesman, and the best hope for promoting a modicum of calm. The latest events now suggest that hope is a chimera — and the administration and those other powers cannot look away from Haiti’s travails.