(CNN) -- A federal grand jury investigation has been launched into the West Virginia chemical spill that left 300,000 people unable to use their water supply, CNN learned Tuesday.
Sources familiar with the grand jury's activities tell CNN that subpoenas have been issued requiring testimony for what one federal official confirms is a criminal investigation.
Meanwhile, an independent water test conducted at CNN's request has found trace levels of the chemical 4-methylcyclohexane methanol, or MCHM, remain in both untreated river water and tap water from two homes in Charleston.
The results by TestAmerica found the chemical is within the safe level of 1 part per million set by the Centers for Disease Control and Prevention; whether that level is safe is disputed.
The test result detected MCHM in the river at .42 parts per billion; residence #1 at 1.5 parts per billion; and residence #2 at 1.6 parts per billion. The amounts in the residences are more than double that found in the river, but still considered a "trace" amount and within safe limits as set by the CDC.
According to a source familiar with the probe, the grand jury investigation has been under way since just after the spill at the Freedom Industries chemical storage facility on the Elk River in Charleston. The spill came to light in early January.
On January 9, more than 7,000 gallons of 4-methylcyclohexane methanol leaked into Charleston's water supply from a Freedom Industries storage tank. The result was a do-not-use order that left about 300,000 people in the area unable to drink or bathe in their water, some for more than a week.
Freedom Industries later told regulators that in addition to the methanol compound that escaped from a ruptured tank, a second chemical -- a mix of polyglycol ethers, known as PPH -- was part of the leak.
PPH is not believed to pose any new health hazard for the people of Charleston, the CDC said. But the state environmental agency said failing to accurately report the makeup of the leak is a violation of state law.
The CDC said little is known about the health hazards of PPH, but it appears to be less toxic than MCHM and made up about 5% of the total volume of the leaking tank.
"Given the small percentage of PPH in the tank and information suggesting similar water solubility as MCHM, it is likely that any amount of PPH currently in the water system would be extremely low," it said. "However, the water system has not been tested for this material."
Elizabeth Scharman, West Virginia's poison control director, told CNN last month that MCHM had not been widely studied.
"We don't know the safety info, how quickly it goes into air, its boiling point," Scharman said.
The chemical is used to wash coal before it goes to market to reduce ash. Exposure to it can cause vomiting, dizziness, headaches, diarrhea and irritated skin, among other symptoms, the American Association of Poison Control Centers and CNN's previous reporting shows.