Agents from the FBI were negotiating with the gunman, CNN reported. The FBI did not immediately respond to questions from Reuters.
The Colleyville Police Department posted on Twitter earlier on Saturday that it was conducting SWAT operations on the block where Congregation Beth Israel is located and said that all residents in the immediate area were being evacuated.
The man could be heard having a one-sided conversation in what appeared to be a phone call during a livestream of the Reform Jewish synagogue's Shabbat service. The livestream cut off around 3 p.m. EST (2000 GMT).
Before the Facebook livestream was ended, a man could be heard ranting and talking about religion and his sister, the Fort Worth Star-Telegram reported. The man could be heard repeatedly saying he didn't want to see anyone hurt and that he believes he is going to die, the newspaper said.
Authorities have not yet confirmed his identity, a U.S. official briefed on the matter told ABC News.