IT IS A TIME of grief in Poland, and a time of unusual tension. The body of the Rev. Jerzy Popieluszko, a prominent supporter of Solidarity, was found in a reservoir on Tuesday. His murder has infuriated Solidarity members and other Poles who shared his religious convictions and his dedication to non-violent change.

The killing seems to represent a brutal and provocative assault on the connection between the Catholic Church and the workers of Poland, a connection that produced the Solidarity movement and survived the banning of Solidarity. If the regime of Gen. Wojciech Jaruzelski fails to respond in a suitable way, it will undercut both its approach to reconciliation within Poland and its prospects of repairing Poland's battered ties to the West.

Three low-level officers of the Interior Ministry are said to have confessed to the murder. On Friday, two colonels were detained and a general suspended in the case. But it is still necessary to ask who at a still higher level, in the Polish government or elsewhere, may have planned the crime. And what was the motive? To get rid of a prominent "troublemaker"? To intimidate Solidarity? To provoke a popular response that might be used as pretext for a new crackdown? Or, conceivably, to challenge Gen. Jaruzelski, whose policy -- hard as it is to believe -- is actually regarded as too soft by some elements in the Polish leadership?

In a communist police state, it is almost unprecedented for arrests to be made so quickly and openly in a political murder case. But it is still not clear what is behind the arrests and whether the proposed investigation is or is not a fraud. At every level, the Popieluszko case has an explosive political potential.

Statements by Solidarity leader Lech Walesa suggest that he fully understands the stakes -- for Solidarity and for Gen. Jaruzelski, who is Solidarity's nemesis but at the same time unavoidably its partner in any Polish revival. While giving voice to the emotions of stunned and outraged Poles, Mr. Walesa has counseled calm. "Restrain yourselves in these dramatic days from manifestations of indignation, from demonstrations or other tensions," he said. "Let the silence of this mourning create conditions to start the dialogue for all. . . . We will all meet at the funeral."