(Bloomberg)—Twitter apologized for the way it responded to Rochelle Ritchie, a former congressional press secretary, when she first alerted it about an apparent threat made against her by mail bombing suspect Cesar Altieri Sayoc.

“The Tweet clearly violated our rules and should have been removed,” it said on Twitter, calling it a mistake. “We are deeply sorry for that error.”

Sayoc was arrested and charged in connection with mailing at least 13 suspected explosive devices to prominent Democrats and CNN, U.S. officials said Friday. A Twitter account by the name of Cesar Altieri with the handle @hardrock2016 posted threats, criticized Democratic leaders and distributed pro-Trump messages.

Twitter added in separate tweets that it is investigating what happened and will continue to work on how to handle concerns raised by users. It said Twitter “wants to be a place where people feel safe, and we know we have lot of work to do.”

This comes after Ritchie tweeted a screenshot on Friday of a tweet she received from the Altieri Twitter account. Ritchie tweeted that she was being threatened by Altieri, and Twitter responded by saying it found “there was no violation of the Twitter Rules against abusive behavior.”

She later tweeted a separate response she received from Twitter, asking her to “disregard” its last reply, adding that it was “sent in error.”

Facebook said it found accounts belonging to Sayoc on its main social network and its Instagram service and removed them. “There is absolutely no place on our platforms for people who attempt such horrendous acts,” a spokeswoman wrote in an emailed statement. “We will also continue to remove content that praises or supports the bombing attempt or the suspect as soon as we’re aware.” The company is working with law enforcement on the investigation.

Another post from the Altieri Twitter account was aimed at the handle of former U.S. Attorney General Eric Holder, a target of one of the mail bombs. “Hey slime scum U like make threats run your hole.Do not worry your cover up Fast Furious with your bitch Obama not forgotten about our very close friend of we Unconquered Seminole Tribe.See u soon Tick Tock 4” the tweet read.

The social-media service has been criticized for allowing abusive and toxic discourse. Twitter’s rules “prohibit behavior that crosses the line into abuse, including behavior that harasses, intimidates, or uses fear to silence another user’s voice.” The policy states that users “may not make specific threats of violence or wish for the serious physical harm, death, or disease of an individual or group of people.”

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