In what witnesses described as a huge explosion, a transformer at the Indian Point nuclear power plant just north of New York City caught on fire Saturday, billowing smoke and prompting the operator to shut down the impacted unit.

Onlookers documented the explosion on Twitter:

At a wedding at #monteverde at old stone and just watched an explosion happen at #indianpoint nuclear power plant. Should I be concerned? — chialing sarva (@chialinghsu) May 9, 2015

Large transformer explosion at Indian Point Nuclear Power Plant. Several Westchester County units on scene operating pic.twitter.com/thmY8uNKsv — RocklandFires (@RocklandFires) May 9, 2015

New York Governor Andrew Cuomo said that, in the aftermath of the blaze, firefighters extinguished the flames with a "special kind of foam." The governor voiced concern about the environmental implications of this substance and oil leaks from transformer: "There is a lot of foam and a lot of oil and now we are worried about that leaking in to the Hudson River."

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Operator Entergy corporation claimed on Twitter Saturday evening that the facility is "safe, secure, and stable" following the "unusual event" but provided no information about the cause of the explosion.

The incident was the second shutdown in just three days. "Unit 3 had been shut down Thursday morning for an unrelated issue, a water leak on the non-nuclear side of the plant," the Associated Press reports.

The clean water advocacy organization Riverkeeper announced on Twitter that it is investigating the incident:

Second unplanned shutdown at #IndianPoint in 3 days, following transformer fire. Riverkeeper patrol boat on scene. Check back for updates. — Riverkeeper (@riverkeeper) May 10, 2015

Located in Westchester County, the facility provides electricity for New York City and surrounding suburbs. Residents have repeatedly raised concerns about the safety of the plant, including the storage of dangerous nuclear waste, and some have called for the facility to be shuttered for good.