Despite police insistence that no potential suspects have been ruled out in the early-morning firebombing of a University of Colorado fraternity house last week, the president of the Alumni Interfraternity Council said Tuesday that a rival fraternity did not fuel the attack.

In a letter being sent to the parents of 1,100 fraternity men at CU, AIC president Mike Smith wrote that members of Kappa Sigma were not involved in tossing a homemade Molotov cocktail at 2:15 a.m. Friday into the first floor of the Sigma Alpha Epsilon house at 1146 12th St., or in a brawl that followed.

“A number of sources have reported that either or both of the incidents, the fire bomb or the scuffle, were the result of rivalries between fraternities in Boulder,” Smith wrote. “Nothing could be further from the truth. ...

“Neither the Kappa Sigma chapter nor its members were involved in either incident. Reports of any Kappa Sigma involvement are categorically wrong.”

Reached Tuesday, Smith said he’s confident no other fraternities were involved.

“We’ve probably done more of an investigation than the police have at this point.”

He said the letter being sent to parents is to assure them that the “fraternity system has made large, positive strides” and that “this kind of rivalry is not the normal kind of practice.”

Boulder police spokeswoman Sarah Huntley said investigators are focusing their attention on residents of an apartment building next door to the SAE house, but they have not ruled out the possibility that an existing feud between SAE and members of Kappa Sigma was involved.

“We’re still open to a variety of possibilities,” Huntley said.

Huntley said some residents of the next-door apartment are also members of other fraternities.

Marc Stine, a spokesman for Boulder fraternities contracted by the Interfraternity Council, insisted Tuesday that no formalized rivalry was involved in the firebombing.

“I guarantee you there was not a meeting where some member said ... ‘Let’s go and blow up the SAE house,’” he said.

Stine said there has not been a documented physical confrontation between members of different fraternities at CU since 2003.

“No guy who sleeps in a fraternity house would throw a fire bomb into another fraternity house — unless he’s crazy,” Stine said.

Leaders from both the SAE and Kappa Sigma houses were expected to release a joint statement Tuesday disavowing any rivalry between the houses, but Stine said the SAE national office would not allow it. National SAE spokesman Brandon Weghorst said it is standard policy to route all public comments through the chapter spokesmen.

SAE alumni, undergraduate members and CU are pooling money toward an increased reward for information leading to the arrest and conviction of whoever threw the explosive cocktail.

CU spokesman Bronson Hilliard said the university’s Parent Fund is working to contribute $1,000 to Crimestoppers — an anonymous tips network that offers up to $1,000 for information about crimes. Stine said the fraternity groups are working to add an additional $3,500 to the reward, making the top reward for information about the incident as much as $5,500.

“(The money) demonstrates the sense of community that exists among the different fraternity groups and their alumni,” said Stine, the Greek advocate. “It’s also an expression of their outrage that somebody would try to hurt kids like that.”

Tips can be submitted to Crimestoppers by calling 800-222-8477, or by visiting www.nococrimestoppers.com. Boulder detectives are also asking for information about the incident, and they can be reached by calling 303-441-3333.