The man killed was James S. Wilson, 22, of Alton.
The deputy, whose name was not released, appeared to act appropriately, said St. Charles County Sheriff Thomas Neer.
Neer was not sure when results of a Wednesday autopsy would be available, but said he hoped an internal investigation would be complete by Thursday. Until then, the officer, a three-year veteran of the department, was on administrative duty.
The deputy, working alone, responded at 9:15 p.m. Tuesday to a call from Piasa Pantry, a convenience store in West Alton, Mo., Neer said. Wilson allegedly assaulted his sister at the convenience store, leaving her with injuries to the lip, nose and forehead, he said.
The deputy ordered Wilson to drop to the ground, but he refused and moved toward the officer in an aggressive manner, Neer said. After repeated warnings, the deputy used the Taser to shock Wilson, he said.
Wilson lost consciousness and began to have trouble breathing. The deputy and later emergency medical workers tried to revive him. Wilson was pronounced dead at a hospital a short time later.
"For the family we extend our heartfelt condolences for their loss," Neer said.
Wilson was unarmed, but Neer said a preliminary investigation indicated the deputy acted "by the letter" of department policies and procedures. The Taser itself was also checked and worked properly, the sheriff said.
The department has used Tasers since 2004. Officers undergo eight hours of training before they are issued the stun gun, Neer said.
"I think it's one of the most effective and useful tools that a law enforcement officer working the street has at his or her disposal," Neer said.
"This is the first incident even close to this we've had in our county."
Spokesmen for Taser International Inc. did not respond to requests seeking comment.
Nationally, thousands of police agencies use Taser systems, which give police a way of subduing suspects without using their firearms.
A Justice Department study released in October cited 36 arrest-related deaths after the use of Tasers or similar devices, though more than half were blamed on other causes, including intoxication.
Neer said he was unsure if Wilson had a criminal record, but St. Charles County authorities were not familiar with him.
