WASHINGTON - Palestinian Authority Chairman Yasser Arafat is blocking an investigation to find the killers of three American security guards attacked in the Gaza Strip last year, a senior U.S. official said on Tuesday.

The United States has persistently complained the Palestinians have failed to investigate fully the bombing of a U.S. diplomatic convoy but officials had not directly blamed Arafat for the handling of the probe before.

"There has been no satisfactory resolution of this case. We can only conclude that there has been a political decision taken by the chairman [Arafat] to block further progress in this investigation," Deputy Assistant Secretary of State David Satterfield told lawmakers.

A Palestinian Authority court in March ordered the release of four Palestinians who were charged with involvement in the October strike because of lack of evidence.

The attack strained relations already frayed by Washington's efforts to sideline Arafat and U.S. criticism of the Palestinian Authority's failure to crack down on militants who attack Israelis.

Satterfield's remarks at a congressional hearing on the Middle East conflict add pressure on Arafat, who faces the stiffest challenge to his leadership since Palestinians received a measure of self-rule from Israel a decade ago.

U.S. officials have called the bombing the first fatal attack to deliberately target Americans since the Palestinian uprising began in 2000. The convoy was driving to Gaza City to interview applicants for U.S. Fulbright scholarships.