Though power has been restored to most U.S. Midwest areas hardest hit by the first winter storm of the season, Missouri's utility regulators are frustrated that more has not been done to prevent widespread outages.

Crews have restored power to about 323,000 customers since the Dec. 1 storm, according to the St. Louis-based utility company, Ameren Corp.

Roughly 70,000 Illinois customers remained without power late Tuesday night, estimated company spokesman Leigh Morris.

Some rural areas will not get service until Friday, he said.

Frustrations with the prolonged power outage prompted the Missouri Public Service Commission to summon Ameren's chief operating officer to a meeting Tuesday. Commissioners were not satisfied with Executive Vice President Thomas Voss' explanation that the ice storm was the worst to hit the St. Louis since Dec. 31, 1978, and that most of the damage was caused by trees rooted on private property.

Sixteen deaths in Missouri and Illinois have been blamed on the storm and its aftermath.

Eleven of the confirmed deaths were in Missouri. Nine more deaths in the two states — five in Missouri, four in Illinois — were suspected to be weather-related.

Using portable generators or burning other fuels indoors has killed at least two people and sickened dozens in the St. Louis area. The generators give off lethal carbon monoxide, which can accumulate quickly inside homes.

By Tuesday, at least 43 people had been admitted to hospitals in the St. Louis area because of carbon monoxide poisoning, according to authorities.