Declaring "the mistake is ours to own," Muslim leaders in Philadelphia apologized Wednesday for an event last month at which children were captured on video speaking in Arabic about beheadings and the liberation of Jerusalem's most sensitive holy site.

The Muslim American Society's Philadelphia chapter and the Leaders Academy, a gathering place for cyberschooled children that rents space at the site, issued a joint statement Wednesday that sought to explain how the questionable material found its way into an annual event meant to celebrate diversity in Muslim communities.

A video uploaded to the chapter's Facebook page showed children dancing to a revolutionary anthem often used by Islamist groups, and two young girls reading from a prepared text. One said, "We will chop off their heads, and we will liberate the ... Al-Aqsa Mosque."

The contested site, revered by Jews as the Temple Mount and by Muslims as the Noble Sanctuary, is at the heart of the Israeli-Palestinian conflict. The compound is the third-holiest site in Islam and the holiest site for Jews. It has been a flashpoint of violence in the past.

The Facebook video, which has since been taken down, sparked outrage and an ongoing investigation by the Philadelphia Commission on Human Relations.

"Over the last decade our members have poured their soul and resources to create a harmonious, peaceful and engaged community," the statement said. "We are very sad that within minutes all of this work was tarnished and we realize the mistake is ours to own ... we are deeply saddened to have hurt our partners in the Jewish community and beyond."

The April 17 "Umma Day" event was intended to showcase diversity, with 18 countries represented and children singing songs from various Muslim cultures and wearing customary dress, according to the statement. Officials said a volunteer aide selected the songs out of a desire to represent Palestinian people, "feels terrible she made a mistake" and has since stepped down. A large number of Palestinian immigrants attend the Al-Hidaya mosque at the site.

"The children did not understand this song as their command of Arabic is not advanced," the statement said. The speech, it said, "was likewise a selection that primarily sought to highlight the children's capacity to read and project Arabic rhetoric; however, they have not yet mastered enough grammar to comprehend the words."

The Muslim American Society's lawyer, Timothy Welbeck, said Wednesday he didn't know who chose the speech, or why it was selected.

The Leaders Academy does not consider itself a school — a designation that would require it to register with the state Department of Education — but a gathering place for children following a state-approved cyber school curriculum. The academy supplies tutors to keep the students on task and answer questions, according to Welbeck, and also provides Arabic language and religious instruction.

The Leaders Academy and the Muslim American Society said they have taken steps to prevent a repeat of what they called a "grave mistake," including new operating procedures, a new advisory council and sensitivity training.

"We appreciate the very detailed and thorough statement and apology that was released today," said Nancy Baron-Baer, regional director of the Anti-Defamation League in Philadelphia. "We understand that people make mistakes. With that said ... it's incumbent on the adults in the room to understand what was being said and recognize that words like that are absolutely unacceptable at any time, in any language."

The statement from the Muslim groups also said it has been targeted with abusive phone calls in the wake of the video, and two protests at the building in which crowds shouted "hateful messages." It said security has been increased.