The FBI is struggling to decode private messages on phones and other mobile devices that could contain key criminal evidence, and the agency failed to access data more than half of the times it tried during the last fiscal year, FBI Director Christopher Wray told House lawmakers.

Wray will testify at the House Judiciary Committee Thursday morning on the wide range of issues the FBI faces. One of the issues hurting the FBI, he said, is the ability of criminals to “go dark,” or hide evidence electronically from authorities.