BOULDER, Colo. (CBS4)- The Maxwell Fire burning in Lefthand Canyon was 40 percent contained at 3 p.m. Monday. The fire has burned 100 acres. All evacuations that had been ordered on Sunday were lifted.

The grass fire burning in the 4000 block of Lefthand Canyon Drive started at about 5 p.m. Sunday. Air tankers resumed making drops of retardant Monday morning. A helicopter was making water drops from the air.

An air tanker drops fire retardant on the Maxwell Fire burning in Lefthand Canyon on Sunday. (credit: CBS)

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Fire investigators said the fire started along the side of Lefthand Canyon Road, about a quarter mile from a popular recreation area.

Air tankers were diverted from the Maxwell Fire to the Duckett Fire burning near Westcliffe in southwest Colorado. That fire flared up on Sunday.

Monday’s weather conditions of cooler temperatures and high humidity are helping firefighters contain the fire.

“The humidity is supposed to be a lot higher today than yesterday and the temperature is supposed to be a lot lower, so the incident commander has asked firefighters to do some major, heavy hard work today so that we can get a lot done while the weather’s in our favor,” said U.S. Forest Service spokeswoman Elsha Kirby.

On the ground, about 100 firefighters were battling the blaze, along with six to seven engines. The incident commander said an attempt to start a back burn in the area where firefighters worked to corner the fire into previous burn areas overnight Sunday was successful.

The Maxwell Fire burning in Lefthand Canyon on Sunday. (credit: CBS)

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“The incident commander decided that what he wants to do is beat the fire to the road. So he is going to be conducting a back burn operation and they’re going to try to push the fire to the old Overland Fire and what burned in the Lefthand Canyon fire in March,” said Kirby.

On Sunday evening, many homeowners decided to ignore a mandatory evacuation order, instead preparing to leave if the fire grew closer to their homes.

“Well, it hasn’t jumped the Lefthand Canyon or Lefthand Creek it’s on the north side of it we’ve got two protective barriers before it could come up on our side,” said homeowner Chuck Rose. “There’s really nothing you can do about it except go with the flow.”

(credit: CBS)

More than 300 homes received reverse 911 calls Sunday at about 6:30 p.m. ordering mandatory evacuations. Smoke from the fire could be seen miles away in downtown Denver.

The fire is burning in timber, dead and downed materials. The cause of the fire is unknown and being investigated.

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In March, a wildfire burned 622 acres of National Forest land in Lefthand Canyon, which prompted the evacuation of more than 200 homes. No homes were damaged in that fire.