Gov. Rick Perry urged anyone with information about who killed a Dallas-area district attorney, his wife and an assistant prosecutor to contact law officers as the reward for tips leading to convictions doubled Thursday to $200,000.

Nobody has been arrested in the weekend slayings of Kaufman County District Attorney Mike McLelland and his wife, Cynthia, nor in the Jan. 31 death of Mark Hasse, an assistant DA gunned down as he arrived for work near the county courthouse.

"We will not let this cower us. Texas is a law-and-order state and we will track down and punish those who have committed this crime," Perry said at a news conference where he was joined by the FBI, sheriff's officials and other law enforcement.

The governor said it was "very premature" to speculate on whether a particular group is responsible. No suspects have been publicly named, but speculation has swirled around a white supremacist prison gang known as the Aryan Brotherhood of Texas, which had been targeted by a task force that included McLelland's office.

Perry planned to attend Thursday's memorial for the McLellands, who were found shot to death Saturday in their house near Forney, about 20 miles east of Dallas. Flags were flown at half-staff Thursday outside the courthouse, which closed for the afternoon so co-workers could also attend the service.

The state's $100,000 reward offer matches the one offered by Kaufman County Crime Stoppers after Hasse was slain.

"The public at the end of the day will play the most important part in this," Perry said.

Dallas FBI Special Agent in Charge Diego Rodriguez said federal law officers also sought information.

"At this time we're looking at everything available, every single avenue. We're not leaving any stone unturned," he said.

Kaufman County Sheriff David Byrnes also encouraged the public to offer any tips, "no matter how minor you think it may be."