JERUSALEM?A Palestinian detonated nail-studded explosives on a Jerusalem bus crowded with high school students and office workers Tuesday, killing himself and 19 passengers in the city’s deadliest suicide attack in six years. Fifty-five people were wounded.

The extremist Islamic group Hamas claimed responsibility for the attack?the 70th Palestinian suicide bombing in 21 months. The bomber, a 22-year-old university student, wrote in a farewell note he had tried twice before to carry out an attack.

It was not clear whether the bombing would delay plans by President Bush to issue a major Mideast policy statement, which had been expected by Wednesday. Bush condemned the attack “in the strongest possible terms,” said White House spokesman Scott McClellan. U.S. officials said Bush might send Secretary of State Powell to the Middle East next week, but has not made a decision.