(NEXSTAR) – A messaging app used by teachers and parents in school systems across the country said Wednesday that an explicit image sent to parents late Tuesday was part of a “coordinated attack.”

Teachers from Illinois, Colorado, Kansas, Minnesota, New York, South Dakota, Michigan and other states said the image was circulating at their schools, according to Motherboard.

NBC News said a screengrab from a Florida parent showed the image, which it said was a meme “infamous on the internet” that showed a man “engaged in an explicit act.”

The San Francisco-based company said in a news release that the platform itself was not hacked and that they suspect someone used compromised email-password combinations to log into Seesaw accounts.

“We have no evidence to suggest this attacker performed additional actions or accessed data in Seesaw beyond logging in and sending a message from these compromised accounts.

Seesaw did not immediately return Nexstar’s requests for comment on the number of users who might have received the image.

On the company’s website, Seesaw says that it is used by “over 10 million teachers, students, and family members every month across more than 75% of schools in the U.S.”

Troy City School District in New York warned parents Wednesday of “unauthorized messages” that could contain inappropriate photos or links.

“Please do not read or click any links until Seesaw has resolved the issue,” the District said on Facebook. “We will send an update when we receive it.”

Seesaw blamed a “credential stuffing” attack, which involves bad actors using stolen login credentials to attempt to log in to an unrelated system.

Seesaw temporarily disabled the messaging feature and adjusted its detection and blocking rules to prevent future attacks. The app is urging all users to create unique passwords that aren’t used in other services.