But Robinson's mother, who has spoken twice with her son by telephone since the attack, has no doubts about what motivated the attack.
"I believe it happened because he is a person of color," Tina Robinson said in a telephone interview Friday from her home in Providence. "It was completely unprovoked."
The stabbing took place in Volgograd, an industrial city of 1 million people 900 kilometers (550 miles) southeast of Moscow.
Tina Robinson said her son had developed pneumonia, and said she was trying to arrange his transfer to a Western-style medical facility. "I'm very concerned about the care he's getting there," she said.
The US Embassy declined to comment, citing privacy concerns.
In recent years Russia has seen a rising number of attacks against members of non-Slavic ethnic groups, particularly darker-skinned migrants from the Caucasus region and Central Asia. African students and immigrants are also frequent targets of attacks, but attacks on Westerners are rare.
Two Tajik men were attacked in a town north of Moscow last week. One was beheaded and Russian media reported his head was found 12 miles (20 kilometers) away. An obscure nationalist group claimed responsibility in an e-mail to the Sova hate-crime monitoring group.
Tina Robinson said she was unaware of Russia's troubles with racism when her son left for a year abroad. "If I had any inkling that there was any possibility of this happening, I would have tried to dissuade him," she said.
The victim's mother and police gave slightly differing accounts.
Smolyaninova said three men approached Robinson at about 6 p.m. in a dark street far from his host family's home. The assailants stabbed Robinson twice in the chest, she said.
Tina Robinson said her son had just finished working out at a gym and was headed for a bus stop when a single stranger approached and punched him. Robinson punched back, his mother said. The attacker then pulled a knife and stabbed Robinson in the chest and side, she said.
Relatives said Robinson, a graduate of East Providence High School in Rhode Island, was three months into his stay. He was studying Russian on a program arranged by the American Field Service, or AFS.
A woman who answered the phone at AFS's Moscow offices said no one could comment. She declined to give her name.
