A Massachusetts man was found to be carrying a gun after attending a tour of the U.S. Capitol as he left the nearby Library of Congress on Tuesday, police officials told ABC News on Thursday.

Authorities in Washington were alerted by police in Carlisle, Massachusetts, on Monday that a man with a gun who had expressed suicidal ideations on social media was headed to Washington.

The U.S. Secret Service and Washington, D.C. Metropolitan Police found the man in a Washington hotel early Tuesday morning and interviewed him, sources said. They searched for a gun and found no weapon and no further action was taken at that time, they said.

Later that day, the man went through a Capitol Police magnetometer screening and entered the Capitol Visitor Center. The magnetometer sounded an alarm and "an officer performed a secondary hand search, and the man was let into the building," according to a statement from U.S. Capitol Police.

After getting past security, Capitol Police were alerted to his presence and issued a bulletin to be on the lookout for the man. They located him after he had completed the full tour of Congress outside the Library of Congress a short time later and found that he had a 9mm handgun in his waistband, law enforcement sources said.

Authorities said the suspect was arrested for "Unlawful Activities, Carrying a Pistol Without a License, Possession of an Unregistered Firearm, Unlawful Possession of Ammunition, and Resisting Arrest." The case is currently with the U.S. Attorney's Office, the Capitol Police statement said.

The officer who performed the screening at the magnetometer at the Capitol Visitor Center "is suspended while the USCP's Office of Professional Responsibility is conducting an administrative investigation into the officer's performance of that search."

There is no indication he intended to cause any harm to harm the Congress, according to Capitol police. But sources told ABC News that a man who was possibly suicidal was able to take a full tour of Congress with a gun with members of the public and Congress nearby.

"A full review of this incident has already been ordered, as well as mandatory refresher training on security screening, so this never happens again," Capitol Police said in their statement.

Members of Congress were briefed on the situation on Thursday.