U.S. law enforcement officials expressed concern Wednesday about the growing use of encrypted communication and private messaging by supporters of the Islamic State, saying the technology was complicating efforts to monitor terror suspects and extremists.

The officials, appearing before the House Homeland Security Committee, said that even as thousands of Islamic State supporters around the world communicate in public view on Twitter, some are exploiting social media platforms that allow them to shield their messages from law enforcement.

"There are 200-plus social media companies. Some of these companies build their business model around end-to-end encryption," said Michael Steinbach, head of the FBI's counterterrorism division. "There is no ability currently for us to see that" communication, he said.

Asked later in the hearing if he thought the technology companies were being unhelpful, Steinbach replied, "The companies have built a product that doesn't allow them to help."

He said he was concerned that evolving technologies were outpacing laws that allow law enforcement to intercept communications by suspects.

John Mulligan, the deputy director of the National Counterterrorism Center, testified that one of the two men involved in an attempted terror attack in Garland, Texas, last month urged fellow Islamic State supporters before the shooting to move their communications to private Twitter messages. Those men, Elton Simpson and Nadir Soofi, were shot by police outside a provocative Prophet Mohammed cartoon contest. No one else was killed.