On the morning of June 16, 1991, William McCullum Jr., a 33-year-old security guard, was ambushed and killed inside the United Bank Tower in Denver after responding to a freight-elevator request. The attacker used a false identity to gain access to a secure area, took McCullum’s electronic pass card and keys, and hid McCullum’s body in a storage room before moving to the bank’s basement guard station and vault.

The same assailant then killed three other guards—Phillip Mankoff, Scott McCarthy, and Todd Wilson—and stole about $200,000 from the cash vault in what became known as the Father’s Day Bank Massacre. Authorities arrested a primary suspect, James W. King, a retired Denver police officer and former bank guard, on July 4, 1991; he was tried on four counts of murder but acquitted after a jury deliberated nine days. The stolen money was never recovered and no other arrests have been announced.

McCullum and his colleagues’ murders remain officially unsolved. If anyone knew McCullum, his shifts at the bank, or saw suspicious activity at the United Bank Tower that day, please share what was seen or remembered.