Published: May 11, 2014 10:45 PM EDT

BAUCHI, Nigeria (AP) - There's still no sign of the Nigerian schoolgirls who were kidnapped by Boko Haram militants.

But one of the teenagers who escaped from the Islamic extremists who abducted more than 300 girls says the kidnapping was "too terrifying for words," and she is now scared to go back to school.

Sarah Lawan, a 19-year-old science student, spoke Sunday as Nigerians prayed for the safety of the 276 students still held captive. Their prayers were joined by Pope Francis.

Lawan says more of the girls could have escaped but that they were frightened by their captors' threats to shoot them.

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.

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