Two policemen were killed Monday during a tense standoff with a barricaded gunman who was shot dead when officers stormed his apartment in a bid to end the siege, authorities said.

The gunman, identified as Charles Knowles, 50, had a "history of mental instability," according to a Detroit police spokesman. But the precise reasons for his actions Monday were not immediately clear.The dead policemen were identified as Lt. James Schmit, 41, and Officer Frank Walls, 39.