DETROIT -- A man carrying a handgun in a paper bag fatally shot a federal officer at a security checkpoint Friday when he was told he could not take the gun into the building housing the FBI, authorities said.

The gunman then was shot and wounded, FBI spokeswoman Dawn Clenney said. His condition was not immediately known.

The man entered the lobby of the Patrick V. McNamara Federal Building and put the bag on a counter by the metal detector, Clenney said. After his request to take the gun inside was refused, he reached in the bag and fired, she said.

A witness, Joel Vander Linden of Warren, said he was waiting in line at the metal detectors at the building when he heard a man behind him "complaining loudly ... cussing."

Vander Linden said as he going to the elevators, he heard a "boom," but was rushed out the door before he could see anything else.

Another witness, Henry Verner, said he heard three or four shots and people yelling.

"There was a lot of unrest and people were scurrying to protect themselves," he said. He said he later saw someone lying on the floor who was shot, but couldn't describe the person.

The agent was taken by ambulance to Detroit Receiving Hospital.

A second man, apparently the shooting suspect, was taken out of the building on a stretcher with his neck in a brace and a pile of bandages on his abdomen.

An ambulance left under police escort to an undisclosed hospital. The building was evacuated and the area was cordoned off.

Besides the FBI, the building has offices for agencies including the IRS, Veterans Affairs, the National Labor Relations Board and Housing and Urban Development.

The slain officer, 36, joined Federal Protective Services in 1998, said David Wilkinson, spokesman for the General Services Administration. Previously, he was a military policeman with the Marines and a police officer with the Department of Veteran's Affairs, Wilkinson said. Authorities did not release the officer's name.