KIEV, Ukraine — Volodymyr Danyluk was a Soviet Army veteran who joined demonstrations against Ukraine’s government last year. He was 55 years old, separated from his wife and mostly out of contact with his family, who saw him on live television during a winter of protests.
Then came the authorities’ crackdown last month in Kiev, the capital. The riot police and demonstrators clashed, scores of people were killed and the government of President Viktor F. Yanukovych fell. Mr. Danyluk disappeared from sight.
In the weeks since, Ukraine’s interim authorities have allowed opposition members to search prisons, morgues and hospitals for their missing. There has been no sign of Mr. Danyluk — or of more than 250 other missing Ukrainians.
After a season of political upheaval here, a gnawing worry persists: What happened to Ukrainians who seemingly vanished in their revolution’s fast-moving tides? Were Mr. Danyluk and the others victims of state repression and criminal activity by the police, or had some of them just drifted back to quiet lives?
“Our mom is worried and calling me all the time,” said Mr. Danyluk’s sister, Galyna Onyshchuk, crying.
In all, 661 people have been reported missing since protests began last December, according to Euromaidan S O S, a volunteer group leading efforts to find the disappeared. The fates of 272 of them remained unknown late last week.
Many people were found in prison cells or hospitals, or resurfaced on their own, said Vitaliy Selyk, a Euromaidan S O S coordinator. Some cases were caused by breakdowns in communications, including people who lost cellphones or ran out of credit on SIM cards, he said.
A few of the missing were people estranged from families and whose recent silence was by choice. Mr. Selyk said he expected that most of the remaining cases would be solved and that the missing would turn up.
But beneath that hope lies the grim concern that many Ukrainians may have disappeared after being seized by the Berkut riot police unit, by pro-Russian provocateurs or by unofficial forces that worked to keep Mr. Yanukovych in power.
This fear, cited almost universally by the opposition, is rooted in two particular cases.
The first was the killing of Yuriy Verbytskyi, a seismologist and an opposition activist, who was found dead in January in the forest near Boryspil after being abducted from a Kiev hospital.