It is not known how many suspected Boko Haram members are detained by security forces. Hundreds were killed last month when Shekau's fighters stormed the military's main northeastern barracks in Maiduguri, the birthplace of Boko Haram and the headquarters of a year-old military state of emergency to put down the 5-year-old Islamic uprising.
In the video, two of the girls were singled out for questioning.
"Why have you become a Muslim?" one girl, who looked to be in her early teens, was asked.
"The reason why I became a Muslim is because the path we are on is not the right path," the girl said, nervously shifting her body from side to side, her eyes darting back and forth.
"We should enter the right path so that Allah will be happy with us," added the girl, who said her name had been changed to Halima because she had converted from Christianity to Islam. Like the other girls, she wore a bulky gray hijab that covered her body from head to toe, revealing only her face.
A second girl, who appeared to be in her mid-teens, was asked if she or any of the others had been mistreated. No, she said, adding that they had experienced nothing "except righteousness."
As the girls chanted Islamic verses, some clasped their hands together in what appeared to be the Christian style of prayer before quickly turning their palms upward, as Muslim worshippers do.
The girls' families have said most of those seized April 15 from a school in the northeastern town of Chibok are Christians.
It was impossible to fully authenticate the video, though parents were trying to turn on a generator in Chibok, hoping to watch the video and identify their daughters, said a town leader, Pogu Bitrus.
"There's an atmosphere of hope — hope that these girls are alive, whether they have been forced to convert to Islam or not," he told The Associated Press by telephone. "We want to be able to say, 'These are our girls.'"
The video showed about 100 girls, indicating they may have been broken up into smaller groups as some reports have indicated.
Fifty-three girls managed to escape and 276 remain missing, police say.
Bitrus said vegetation in the video looked like the Sambisa Forest, some 20 miles (30 kilometers) from Chibok, where the girls were believed to have been spirited away.
In Washington, White House spokesman Jay Carney said U.S. intelligence experts were "combing over every detail" of the latest recording. He said administration officials have seen the video and "have no reason to question its authenticity."
In a video last week, Shekau threatened to sell the girls into slavery. It arrived amid unverified reports that Christians among the students had been forced to convert to Islam and that some were taken to neighboring Cameroon and Chad, where they were forced to marry their abductors. Boko Haram means "Western education is sinful."