(CBSNews.com) - The United States is now directly involved in the search for more than 200 Nigerian girls kidnapped from their school.
Manned American spy planes are combing Nigeria for any sign of the girls, who were abducted nearly four weeks ago from the remote town of Chibok.
On Monday, for the first time, their kidnappers put out video of the girls - and made their first demand.
Boko Haram wants the Nigerian government to release members of the terror group it is holding, or, it says, the girls will never be seen again.
Surveillance missions flown by manned aircraft have already begun and the Pentagon is considering adding unmanned drones to the search.
The nearest drone base is in the country of Niger, next door to Nigeria. They were sent there last year to fly north and search for militants operating in Mali. To aid in the search for the girls, the drones, which are unarmed, would have to fly hundreds of miles east over the part of Nigeria where Boko Haram is based.
At the same time, U.S. intelligence is analyzing the newly released video of the girls in an effort to match the surroundings to a more precise location. But the girls could have been moved since that video was shot and U.S. intelligence has indications they are now being held in separate locations.
None of the intelligence collected by the manned aircraft has been shared yet with the Nigerians because of legal restrictions on how much help the U.S. can give to a military that has a record of human rights violations.
The U.S. is sending 30 advisers from the departments of State and Defense and the FBI to assist in the search.
The video released Monday shows more than 100 girls sitting together, dressed in traditional Muslim clothing, reciting Islamic prayers.
The girls appear anxious. Several of them were singled out to answer questions from a man off camera.
He asks one girl why she has converted from Christianity to Islam. She says, "Because the path we are on is not the right path."
There is anguish in Nigeria but also hope. It's not known when the video was shot, but it was the first sign that the schoolgirls were alive after their capture.
Boko Haram leader Abubakar Shekau also appears in the video. He says the girls will not be seen again unless they are swapped for his fighters being held as prisoners in Nigerian jails.
This is not the first time Boko Haram has kidnapped young girls.
Hannah, who was kidnapped last year, was held hostage for more than two months in a Boko Haram stronghold in the forest. She told CBS News she was beaten regularly during this time for refusing to convert to Islam.
"They tied a rope around my neck and pulled me on the ground," she said, "while they kept telling me that they would slaughter me."
Hannah said she was taught how to handle a gun and forced three times to go on raids with the militants.
"They took me along with them," she said, "strapped bullets to my back and instruct me to lie down whenever they exchange gunfire with soldiers."
Since the video has been released, the Nigerian government has been showing it to the parents of the missing girls to see if they can make any identifications.
Asked how the Nigerian government would respond to the video of the girls, Interior Minister Moro Abba told CBS News the government does not negotiate with terrorists. But he also said a process of dialogue continues. So they haven't closed the door on negotiating a release.