Story highlights The 500 denials represent almost 7% of all applicants

The US allowed more than 15,000 Syrian refugees in 2016, many of whom are under the age of 14

Washington (CNN) The US government has denied more than 500 Syrian refugee applications since 2011 because applicants had known or suspected terrorism ties, according to House Homeland Security Committee Chairman Michael McCaul.

McCaul cited the numbers he learned from a briefing by law enforcement and intelligence agencies in a letter this week to the acting director of National Intelligence, Michael Dempsey. CNN has obtained the letter.

The 500 denials represent almost 7% of all applicants. McCaul also said he was told in a briefing that "several hundred additional Syrian cases are on hold pending final review for denial on national security grounds."

The number of terror-related denials, McCaul said, raised new concerns because federal agencies that oversee refugee resettlements are reinvestigating the backgrounds of a number of refugees allowed into the US from Syria.

A technical glitch allowed a small number of refugees to be resettled without a complete check of data on intelligence databases, according to a US official briefed on the matter. It's not clear how long the glitch affected vetting of refugees and how many are being reinvestigated.

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