A leading environmental group says House Republicans have "zero" authority to investigate them or the 17 Democratic state attorneys general who are pursuing a case against Exxon Mobil over climate change.

House Science, Space and Technology Committee Chairman Lamar Smith, R-Texas, announced Tuesday that he will hold a hearing Sept. 14 to define the committee's oversight authority to investigate the attorneys general and aligned environmental groups.

"Maybe instead of this buffoonery, the House science committee could call on, you know, a scientist, to re-explain the threat of climate change and the role of the fossil fuel industry in causing the crisis," said Jamie Henn, a spokesman for the group 350.org. "Rep. Smith sounds like he could use a refresher course."

Smith argues that the climate probe being directed at Exxon and conservative think tanks is an affront to free speech, meant to chill debate about global warming.

The group 350.org, which responded to the hearing announcement early Wednesday, balked at Smith's claim of authority to go after them. The group was recently subpoenaed by the committee, in which Smith asked for all correspondence between the group and the state AGs regarding the Exxon investigation.

The environmental group is a leading proponent of ending all fossil fuel development in the country, referred to as the "keep it in the ground" movement. It is also part of a protracted effort to stop Republican nominee Donald Trump from being elected in November. The group, with several others that were subpoenaed, refused to comply.

"Rep. Smith has zero authority or cause to subpoena us, the attorneys general or any other groups looking to uncover the truth about Exxon's climate lies," Henn said.

The Democratic attorneys general, led by New York Attorney General Eric Schneiderman, started their investigation earlier this year into news reports that Exxon Mobil had covered up studies by its own scientists that showed climate change would harm its business.

Schneiderman and other attorneys general began subpoenaing the oil giant with any groups it may have had affiliation with over the years.

The attorneys general want to show that Exxon officials committed fraud by denying the public information regarding a potentially harmful public health threat.