The rules governing how the board investigating the Columbia accident conducts its probe were formally revised Tuesday by NASA to reflect the group's independence from the space agency.

The changes to the board's operating charter come amid criticism by some lawmakers that the board is too closely tied to and dependent on NASA.

"These charter provisions and others clearly demonstrate that your Board can and must act independently," NASA Administrator Sean O'Keefe wrote in a letter Tuesday to Ret. Admiral Harold Gehman, who heads the investigative panel.

O'Keefe had said last week he would be altering the charter.

Language was changed throughout to show the group has final authority over NASA personnel it receives help from; can use non-NASA personnel; and can release information and schedule activities as it deems appropriate and not according to NASA policies.

The board was also given no deadline for providing a final report, which will be immediately released to the public once completed. Originally, the group was given 60 days to finish its work.

The board began its work within hours of Columbia's breakup on Feb. 1. All seven astronauts aboard were killed.

Rep. Sherwood Boehlert, chairman of the House Science Committee, had said at a congressional hearing last week that he was concerned about NASA interference in the probe and the 60-day time limit and that he wanted immediate public release of any final report.

On Tuesday, spokeswoman Heidi Tringe said the Republican congressman from New York was pleased with the changes made to the charter.