Miles Harrison, 49, drove to work in the town of Herndon on Tuesday with the 21-month-old boy strapped into the back seat, and left the child in the car in the hot sun as the temperature in the vehicle rose to around 55 degrees C (130 degrees F).
Herndon police spokesman Jeff Coulter said: "Mr. Harrison is in the hospital, where he was admitted in a state of shock after the death of the child. We will give him time to recover, and then we plan to arrest him."
Coulter said no charges had been filed against Harrison, but that he would be charged with manslaughter, and if found guilty could face up to 10 years in prison.
The Washington Post earlier quoted Russian Embassy spokesman Yevgeny Khorishko as saying consular officials are trying to establish whether the child, brought over from Russia three months ago, still had Russian citizenship.
The paper said Harrison, the managing director of a real estate consulting firm in Herndon, was supposed to take the child, named Chase, to a day care center on Tuesday morning.
The foster father went straight to work, leaving Chase in the SUV with tinted windows until late in the afternoon, when a passerby saw the child and alerted the office receptionist.
The paper quoted a worker in the same building who saw paramedics placing the child's body in a bag as saying: "That's a terrible death... with the heat. When I got in my car at 1, my steering wheel, you couldn't touch it."
The incident is likely to renew calls in Russia for tighter controls on adoptions following several high-profile scandals, notably the killing of a two-year-old girl from Siberia by her adoptive mother in the United States. The woman, Peggy Sue Hilt, was sentenced to 25 years in prison in May 2006 for beating the child to death.
Around 120,000 Russian children were adopted both in Russia and abroad in 2007, a 6.4% increase on 2006, according to the Science and Education Ministry.
