Tornadoes that spun across the Oklahoma-Missouri border killed at least 18 people as severe storms raked the nation's heart Saturday, injuring many and mangling buildings in the storm-weary region.

At least 12 people were killed after severe storms spawned tornadoes and high winds across sections of southwestern Missouri, the State Emergency Management Agency said. Ten of the dead were killed when a twister struck near Seneca, near the Oklahoma border.

"There may be more — I hope not," said SEMA spokeswoman Susie Stonner late Saturday night.

At least six people were killed earlier in the day as the tornado flattened the northeastern Oklahoma town of Picher, authorities said.

"They're going over the hard-hit area and turning over everything and looking," Stonner said of emergency workers' search for victims and assessment of damage. "It's hard to do in the dark."

The number of injuries across the area was not immediately available, though The Joplin (Mo.) Globe reported that more than 90 people from that region were being treated at Joplin hospitals.

The tornado in Picher (pronounced PITCH-er) — a depressed and pollution-scarred mining town that many residents had already fled — caused major damage in a 20-block area, said Oklahoma's Emergency Management spokeswoman Michelann Ooten.

"I know they are going through the rubble, trying to find people missing," she said. "There are numerous injuries."

Gov. Brad Henry issued a statement saying a major emergency response was under way. He planned to visit the area Sunday.

"Our thoughts and prayers are with the people of Picher and all of the other Oklahoma communities that have been impacted by the latest wave of severe weather," Henry said.

Television footage showed some destroyed outbuildings and damaged homes west of McAlester and near Haywood. At a glass plant southwest of McAlester, the storm apparently picked up a trailer and slammed it on top of garbage bins.

"These are rural areas that we are in," Pittsburg County Undersheriff Richard Sexton told KFOR-TV in Oklahoma City. "These are good people coming together at this time."

In storm-weary Arkansas, a tornado collapsed a home and a business, and there were reports of a few people trapped in buildings, said Weather Service meteorologist John Robinson.

Central Park Elementary School in the northwest Arkansas city of Bentonville had roof and window damage, and damage was also reported at Pine Creek Center School.

The storms remained active into the night as they swept eastward, with watches and warnings abundant across a wide swath of the Plains and South.

Rescuers freed a man trapped in his vehicle in western Tennessee after a tree fell on it during thunderstorms, Memphis firefighters said.

Memphis authorities say they've received reports of power lines and trees down, but there have been no injuries.

Tornadoes killed 13 people in Arkansas on Feb. 5, and another seven were killed in an outbreak May 2. In between was freezing weather, persistent rain and river flooding that damaged residences has slowed farmers in their planting.