Olivet College has reopened a residence hall where dozens of students were sickened with symptoms that included headaches, dizziness and nausea.

Maria Davis, provost and dean at the college, told the Lansing State Journal that school officials Thursday reopened the Blair Hall men's dormitory where most of the 55 affected students live. The cause of what sickened the students on Tuesday and Wednesday is under investigation.

Emergency crews responded and symptoms appeared to be consistent with carbon monoxide exposure. The school says Blair Hall has been tested, however, and authorities haven't found elevated levels of the gas. The fire department says it's not carbon monoxide poisoning.

The school said that the ill students were back on campus Thursday after being evaluated at hospitals.