WASHINGTON  The Obama administration has decided to continue to imprison without trials nearly 50 detainees at the Guantánamo Bay military prison in Cuba because a high-level task force has concluded that they are too difficult to prosecute but too dangerous to release, an administration official said on Thursday.

However, the administration has decided that nearly 40 other detainees should be prosecuted for terrorism or related war crimes. And the remaining prisoners, about 110 men, should be repatriated or transferred to other countries for possible release, the official said, who spoke on the condition of anonymity because he was not authorized to speak about the numbers.

There are just under 200 detainees left at the detention center.

President Obama established the task force shortly after his inauguration last year as part of his administration’s effort to deal with the detainee issues left behind by the Bush administration. It was facing a deadline of Friday to complete its work.

For the past year, national-security and law-enforcement officials under the direction of Matthew G. Olsen, a Justice Department lawyer, have been pulling together scattered files for each detainee at Guantánamo. They evaluated any evidence against each man, the perceived threat he might pose if released, and the possibility of successfully prosecuting him.