A two-year-old boy has died and his older brother is seriously ill in hospital after their mother left them alone in an SUV with the engine running, authorities said.

Heather Jensen, 24, found her sons on Tuesday after leaving them in the vehicle for 90 minutes while she met up with a male friend in a parked car next to her own.

Mrs Jensen called 911 to say her two-year-old son William was not breathing and had no pulse from the car park close to Powderhorn Mountain ski resort in Grand Junction, Colorado.  

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Tragedy: Two-year-old William (left) died on November 27 in Colorado while his four-year-old brother Tyler (right) died a week later in hospital. The two had been left by their mother in an SUV with the engine running for 90 minutes

Widowed: Heather Jensen, 24, lost her 26-year-old husband Eric in a car accident last year

She also told dispatchers at 7pm on Tuesday that four-year-old Tyler was barely breathing.

William was pronounced dead at the scene while Tyler was flown to St. Mary’s Hospital before being transferred to Children’s Hospital in Denver. Tyler remained in critical condition on Wednesday.

Authorities say the investigation is continuing and no charges have so far been filed. The Mesa County sheriff’s department were examining the SUV, including the emissions system for the presence of carbon monoxide.

The boys’ grandfather Robert Mathena told Fox 31 that he believed the boys had carbon monoxide poisoning. William’s cause of death has not been released.

Mr Mathena said that Tyler’s condition is improving and that ‘he’s opening his eyes, looking around’.

Devastating loss: The young mother lost both her sons in the tragic accident last year

Deadly: Heather Jensen dialed 911 on November 27 when she realized that her youngest son was not breathing in the back seat

Heather Jensen was widowed after her 26-year-old husband Eric was killed when his car was hit head-on by an SUV on October 10. The other driver suffered minor injuries.

Mrs Jensen told investigators that she had gone to meet a male friend on Tuesday night when the tragedy took place.

According to the Daily Sentinel, she let the boys play in the snow before putting them back in her Toyota 4Runner and turning the engine on.

She then sat in the man’s truck for around an hour and a half, police investigators said.

The 24-year-old, who has a history of taking anti-depressants, is on probation for a domestic violence assault against her late husband earlier this year.

In love: Mr and Mrs Jensen with their youngest son William. The young mother lost both her sons and husband in a matter of weeks

Adorable: Tyler was in a Denver hospital recovering from his injuries but William (right) was pronounced dead at the scene

Deaths: Mr Jensen was killed in October when his car was hit head-on by an SUV. His sons William and Tyler also died in a vehicle-related death

Emergency: Police were still investigating the tragic incident but had so far made no arrests