All three men in the Jeep have been questioned in connection with the arson and are considered suspects, though they have denied involvement, the police said.

Mr. McCraw said the arsonist is believed to be a fourth person, whose shadowy figure can be seen in video taken by another camera the night of the fire. Released last week, the video shows the figure tossing a blazing gasoline bomb onto the porch of the mansion, then sprinting away. A third camera on an adjacent street captured a grainy image of his face. The police enhanced the picture electronically and released a sketch based on it.

The fire destroyed much of the historic two-story brick home across from the Texas Capitol where governors have lived since 1856. Renovations continue, but Gov. Rick Perry has been moved to other lodgings. For two years, the search for the culprit has been a priority for the rangers and the state police, and the lack of progress had been an embarrassment for the department.

In recent days, rangers in their trademark cowboy hats have fanned out across Austin and penetrated the city’s counterculture hangouts, where the fashion accessories tend toward piercings and tattoos, the music is alternative rock and globalization is a dirty word. Toting pictures of suspects and offering a $50,000 reward for leads, the lawmen have questioned several self-described anarchists about the Affinity group and the identity of the bomber.

Not surprisingly, the investigation has been met with some suspicion. Many self-described anarchists in Austin do not advocate attacking all forms of government, a concept they regard as dated. Their vision of anarchism holds that people should take action themselves to fix social problems. Local anarchists run a recycling center, a food-distribution program, a bookstore, a cafe, and even a thrift shop.

Yet federal agents accused two men from these circles of plotting to make firebombs and hurl them at police cars during the convention. An F.B.I informant from Austin, Brandon Darby, was traveling with the group and told the authorities of the plot, which he had encouraged.