Investigators say the suspect smashed the glass door of the Chabad of Bushwick early Saturday morning.
Rabbi Menachem Heller says children were among at least 20 people inside when the vandal struck.
"We jumped up, I just grabbed the kids and I ran," he said. "We waited a few seconds, and then we noticed two gentlemen in the door through the window, through the glass of the door. We saw them looking in, and then they continued walking."
Police say there is no evidence of anything being thrown, no rock or brick found at the scene. Nothing came through the window, and nothing was found outside.
The incident has not yet been ruled a hate crime, but Rabbi Heller says hate crime detectives came to investigate and that he has his suspicions.
"This is a very warm community," he said. "Bushwick is a very open place, and this is not something typical that should happen over here."
Mayor Bill de Blasio said Sunday that the NYPD will be adding extra security outside the synagogue, as well as other synagogues nearby.
"We are on the trail of anyone who would commit a hate crime, and the NYPD has had a high level of success finding people," de Blasio said.
No one was injured in the incident.
Meantime, clergy in the Brooklyn held a rally in front of the synagogue to show all vandals that "an attack on any House of Worship is an attack on all Houses of Worship."
"We are going to stand with you rabbi and that whatever you need, whatever support you need," Pastor Gilford Monrose said. "We are here to be with you."
The ministers came from churches across Brooklyn, along with clergy council in various police precincts. Their message of unity was aimed at those who would seek to vandalize houses of worship.
"Anyone attacking us at all, that attacks any of these institutions, are attacking us," 83rd Precinct Clergy President Bishop Michael Clark said. "Because it can be us next."
Anyone with information in regard to this incident is asked to call the NYPD's Crime Stoppers Hotline at 1-800-577-TIPS (8477) or for Spanish, 1-888-57-PISTA (74782).
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