Two under-construction apartment buildings blazed from fires only two days apart. One of the fires is now being investigated as a possible arson. 

Aurora Fire Rescue continues to investigate two apartment building fires that set the Aurora sky with plumes of black smoke last week, each rising to the three-alarm level. One fire occurred in the 7300 block of South Addison Court on Thursday , destroying five apartment units in a building under construction. The second was in the area of East Colfax Avenue and Peoria Street on Saturday , causing a fifth-alarm response from local firefighters.

While Aurora Fire Rescue continues to put out “active pockets” of fire in the apartment complex at Colfax and Peoria, detectives are investigating the one on Addison as a “potential criminal act,” according to Aurora Fire Rescue spokesperson Carlos Oropeza during a press briefing on Monday.

The fire at Addison Court raged from around 12 a.m. until 8 a.m. on Thursday, requiring 23 units from Aurora to fight the fire.

The fire at Colfax and Peoria began around 1:30 p.m. on Saturday and required the help of five departments. As of Monday, there was too much destruction and collapse hazards for AFR to send in any personnel for investigations, Oropeza said. 

“I believe the only thing those two have in common as of right now is that they both were under construction,” Caine Hills, the deputy chief with Aurora Fire, told The Denver Gazette’s news partner, 9News. “So the origin of cause continues to be under investigation at both locations.”

On the other side of the metro, a fire broke out in an under-construction apartment building at 38th Avenue and Upham Street in Wheat Ridge around 6 a.m. on Tuesday.

Construction materials inside caught fire, setting off the building’s sprinkler system, according to a social media post by West Metro Fire Rescue. The cause was a “spontaneous combustion caused by rags that had been used for staining,” according to a subsequent post.

The department added that oily rags can self-heat to combustion and ignite if left in a tight pile. Oily rags should be separated, left to dry and then placed in a metal can and filled with water. 

The fire — though smaller than the two in Aurora — marks the third under-construction apartment fire in the past week. 

The Denver Gazette’s news partner, 9News, contributed to this story