Data withheld from an annual report on terrorism by the US state department show a sharp increase in attacks in 2004, a top Democratic lawmaker says.
The Madrid bombings were among the most deadly attacks of 2004
Henry Waxman, citing official briefings given to congressional aides, said the number of "significant" attacks had risen more than three-fold in a year.
He said there were about 650 such attacks in 2004 - up from 175 in 2003.
The California congressman urged the administration to release the figures.
The state department has said it will stop providing them. It admitted to mistakes in last year's report.
In June 2004 the department was forced to double its original estimate of terror victims in the previous year.
It said 625 people had been killed in attacks worldwide in 2003 - not 307 as stated in the annual Patterns of Global Terrorism report published two months earlier.
US DEFINITION OF TERRORISM
Washington has long defined "terrorism" as politically motivated violence against non-combatants
Attacks targeting US or other forces in Iraq are not included
"Significant" terror attacks result in loss of life, serious injury, or major damage
Last week the department announced that it would no longer publish statistical data, but that would be done by the newly created National Counter-Terrorism Centre (NCTC) - an arm of the Central Intelligence Agency.
The NCTC on Wednesday released the figures separately from the annual State Department report, according to the AFP news agency.
The NCTC said that there were 651 terrorist attacks in the world in 2004 in which 1,907 people died.
On Tuesday Mr Waxman accused the state department of concealing an increase in terror attacks.
He said congressional briefings by federal officials pointed to a dramatic rise in the number of "significant" terrorist attacks - those that result in loss of life, serious injury or major damage.
In a letter to Secretary of State Condoleezza Rice circulated by his staff, Mr Waxman said that even the latest figures may be too low.
He said many incidents most Americans would regard as terror attacks were excluded from the data because they did not meet the state department's definition.
"The large increases in terrorist attacks reported in 2004 may undermine administration claims of success in the war on terror, but political inconvenience has never been a legitimate basis for withholding facts from the American people," the letter added.
Last year's original report said there had been a total of 190 terror acts around the world in 2003 - pointing to a steady decrease over three years.
The administration seized on this and other numbers as proof that its war on terror was succeeding.
In the revised report the department said there had in fact been 208 attacks - an increase from 2002.
Then-Secretary of State Colin Powell said there had been no attempt to deceive people, and blamed clerical and administrative errors.