The National Transportation Safety Board also said in a statement that steering, engine and navigation equipment aboard the Andrew J. Barberi worked properly in post-crash tests, putting clearer emphasis on actions of the crew.

The safety board said the captain, previously identified as Michael Gansas, failed to appear for a scheduled interview on Tuesday to discuss the Oct. 15 disaster. The subpoena seeks another interview on Wednesday.

Questions remain about the captain's oversight of his assistant, Richard Smith, who was at the controls.

Reports have said that Smith may have become temporarily incapacitated before the 310-foot ferry with 1,500 people aboard struck the Staten Island pier.

The safety board has interviewed crew members, passengers -- including some who were injured -- emergency personnel and ferry company officials.

"The safety board will continue to aggressively pursue safety and we are moving forward with our investigation," Ellen Engleman, the board's chairman, said. "We hope to continue to get full cooperation from all parties."