The attack in Gaza provoked a level of confusion in Washington rarely seen in the Bush administration.
Condoleezza Rice voiced a tough line against Hamas following the attack
Nobody in the administration has condemned it.
Indeed first thing on Monday morning, the National Security Adviser, Condoleezza Rice, seemed close to endorsing the killing.
Hamas was a terrorist organisation, she said, and Sheikh Yassin was heavily involved in terrorism.
A few hours later, the State department spokesman, Richard Boucher, struck a very different note.
He said the US government was deeply troubled by the attack. Even then, it took a while for the White House to echo that concern.
Contradictions and complications
The problem for the US administration is that it has two contradictory policies.
On the one hand, Washington has always opposed targeted killings or assassinations by Israel.
On the other, the instinct of this administration is to support the Israeli government in what Israel says is its part of the war on terror.
And there are more complications.
President Bush doesn't want friction with Israel as he campaigns for re-election
Washington has been working closely with Ariel Sharon to make sure that his plans to withdraw from Gaza and parts of the West Bank do not produce chaos.
Also, the Arab League is about to take up President Bush's call for political reform in the Arab world.
Sheikh Yassin's killing must surely complicate both issues.
One more consideration for President Bush is that he certainly does not want to get into a public disagreement with the Israeli government, as he campaigns for re-election.
That uncertainty has given Ariel Sharon enough scope to carry out this attack in the knowledge that any criticism from Washington is not likely to threaten his position.