Shares of natural gas company NiSource dropped 11.8 percent Friday after a series of gas explosions left several homes near Boston demolished or engulfed in flames.

Investigators said the explosions took place Thursday because of "over-pressurization of a gas main" that belongs to Columbia Gas of Massachusetts, a NiSource subsidiary.

The news led the stock to its worst day since Nov. 2, 2001, when it dropped 12.3 percent. The stock also erased its 9.4 percent gain for the year and the company's market cap dropped from $10.2 billion to $8.9 billion. However, NiSource shares, however, remained up nearly 6.4 percent over the past six months.

The explosions killed at least one person, injured more than a dozen others and led to the evacuation of thousands, officials said.

"We are focused on providing as much support as possible to our customers, residents and communities," NiSource said in a statement. "We expect this will be an extended restoration effort, and we will work tirelessly to restore service to the affected customers."

Massachusetts Governor Charlie Baker declared a state of emergency for the area while expressing frustration with the utility company whose gas pipeline exploded.

On a number of very significant issues, we heard one thing, then something else happened. We heard one thing, then something else happened, Baker said at a press conference.

—Reuters contributed to this report.