The failure to rescue those who remain captive four weeks later has attracted mounting national and international outrage. Last week, Nigeria was forced to accept international help in the search, after ignoring offers for weeks.
More experts are expected in Nigeria to help rescue the girls, including U.S. hostage negotiators and others from Britain, France, China and Spain.
"I am pained that my other colleagues could not summon the courage to run away with me," Lawan said. "Now I cry each time I come across their parents and see how they weep when they see me."
Police say 53 students have escaped. Nigeria's homegrown Boko Haram terrorist network is threatening to sell those who remain in captivity into slavery.
In churches across the nation, Nigerians prayed for the girls, whose plight has brought together ordinary people in a year that had seen growing dissension between Muslims and Christians, disagreements exacerbated by the increasingly deadly attacks of the Boko Haram terrorist network. Africa's most populous nation of 170 million has almost equal numbers of Christians and Muslims.
The Rev. Stephen Omale prayed at a church in Abuja, the Nigerian capital.
"Wherever they are, God will bring them out in his own mercy, he will see that they are brought out safely, without harm and also that this act will bring an end to all those who are perpetrating these acts," he told congregants.
Boko Haram claimed two bomb blasts that killed about 100 people and wounded more than 200 in the capital last month.
Britain, Nigeria's former colonizer, has said it hopes to help rescue the girls and to halt the 5-year-old Islamic uprising that has killed thousands of Muslims and Christians and has driven some 750,000 people from their homes.
U.S. Defense Secretary Chuck Hagel cautioned that it is "going to be very difficult" to find the missing girls. In an interview with ABC's "This Week" that aired Sunday, he said " It's a vast country... But we're going to bring to bear every asset we can possibly use to help the Nigerian government."
The office of Israeli Prime Minister Benjamin Netanyahu said in a statement that the leader spoke with Nigeria's president and told him: "We are ready to assist in locating the girls and fighting the brutal terrorism inflicted on you."
It did not elaborate on what kind of assistance Israel is offering. Israel has security relations with Nigeria as well as other African nations, including Kenya whom Israel helped with security advisers in the Nairobi mall attack.
A Nigerian security expert warned that militants may have laid land mines to discourage any pursuit, and said strategists may be considering starving them out.
"They may have laid land mines, one cannot rule that out," formed Nigerian Air Force Commodore Darlington Abdullah said on Sky TV. And "even as they go along abducting children, they are also going after food, grabbing food," Abdullah said.