HUMBLE, Texas -- A mother was killed by the family van being driven by her 12-year-old son.

The boy crashed the van through a dental office wall on Tuesday, killing his mother and injuring two others, authorities said.

Lisa Lewis was standing outside the Monarch Dental Associates office in Humble, Texas, when she was killed.

Officials said Lewis had just finished her dental appointment shortly after 2 p.m. and let her two sons, a 12-year-old boy and his 17-year-old brother, get their van so she would not get wet from the rain, Houston TV station KPRC reported.

The mother was outside talking with two office employees.

"They went out to get the van to bring it closer to get mom out of the rain. Instead of the 17-year-old driving the van, the 12-year-old was behind the wheel. When mom noticed it, she was waving her hands, like, 'What are you doing?' And speculation is that the boy panicked. Instead of hitting the brake, he hit the accelerator," said Johnnie Ray Scroggins, Humble spokesman.

The crash pushed his mother through the glass wall and injured two dental workers in the office reception area. The workers are hospitalized in critical but stable condition. They are expected to recover.

"I couldn't believe what I saw. Next thing I knew was he backed out and hit something backing out. I heard a scream," witness Mark Rabinovitch said.

There were no reports of injuries to either of the two boys.

People could only speculate as to why the 12-year-old got behind the wheel.

"Little brother probably saying, 'I can do it. It's not that far. Let me do it. Let me do it.' And older brother said, 'Hey, go right ahead.' And it was unfortunate and a definite accident," Scroggins said.