One of the homes was built with help from the Colorado Republican and his staff.
Allard said he has asked the director of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives to help state and local officials in the case.
The houses were being readied for families to move in this fall. One burned July 12 and the other was destroyed Saturday. A third home nearby being renovated for the city's urban renewal authority was slightly burned.
Neighbors reported seeing a teenager running from the scene of one of the fires, but no suspects have been publicly identified.
``This appears to be a pattern of planned criminal activity that presents a serious threat to the public safety in Greeley,'' Allard said in a prepared statement.
Two contractors have offered to rebuild the Habitat for Humanity homes.
Some neighbors have suggested the fires were set by someone who opposes the project because of fears the housing for low-income families might hurt property values or because a prairie dog colony was wiped out to clear the way for the houses.
Ursula Haney, a single mother of two, is getting ready to move into a home between the two that were destroyed. She said she and her soon-to-be neighbors celebrated July 4 together.
``It's kind of like a family down there already,'' said Haney, a daycare worker and x-ray technician. ``We have five families who have waited two years, and it was getting close. Now someone does this.''
Habitat for Humanity eventually hopes to build 61 homes on the site north of the city over 10 years.
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