( CNN ) -- Helpless . Hopeless . That 's how witnesses described feeling after three people slipped into the foamy waters of the Big Sioux River , in the middle of a Sioux Falls park . The youngest of three -- a 6-year-old boy -- was able to escape the frigid waters alive . But the other two people -- one of them his sister -- who went after him did not survive . `` These people literally jumped in without thinking of their own safety and trying to rescue that child , '' Sioux Falls fire Chief Jim Sideras said . The drama unfolded Thursday evening amidst the 123 acres of Falls Park , which is named -- like the city of Sioux Falls itself -- for the series of picturesque Big Sioux River waterfalls that run through it . According to Sioux Falls police Chief Doug Barthel , the boy had gone into the several-feet-high foam often generated there by the river , and perhaps into the water itself . His sister , who Barthel identified as 16-year-old Madison Wallace , at one point went in after him and ended up falling into the surging water . A bystander , 28-year-old Lyle Eagletail , then went into the river trying to help , said the police chief . He , too , was sucked in -- but not before others at the scene did what they could to save him . One of those who tried was Napoleon Ducheneaux . His shirt muddy , he told CNN affiliate KSFY on Thursday how he and two others had grabbed hold of the man who 'd fallen into the river . `` We had him ... but he just slipped , '' Ducheneaux said . `` I heard him say , 'Hold onto me , I 'll hold onto you . ' That was the last I heard his voice . '' Matthew Krier shared Ducheneaux 's sense of helplessness about not being able to do more . `` Hopeless , I felt hopeless , '' Krier , who watched the scene unfold from a nearby bridge , told KSFY . `` I wish there was something I could have done . '' Authorities from various agencies converged on the scene , their task of finding the two people complicated by zero visibility in the cold , fast-moving , debris-filled waters . Heavy machinery was brought in to break up and remove ice from the river , so that boats and divers could get in , Sideras explained . New water heading into the river was shut off Thursday night , slowing down its flow , but the task nonetheless remained difficult . `` Everybody here is essentially eyes on the water , '' said the fire chief . `` We 're looking for something maybe as small as a knee , a patch of clothing . '' During the day Friday , the rescuers found the body of the rescued boy 's sister . Sideras said he talked with her father afterward . `` They are all in a state of shock , '' the fire chief said . `` Our prayers are with them , and we 're just trying to help them cope as best we can . '' That left authorities with one more person to find . While there 's a limited area as to where he may be , that does n't necessarily mean his body will be easy to spot . `` It 's still going to be an issue of where we 're going to find him , '' Sideras said . Water safety expert : 'Drowning does n't just happen '
A 6-year-old boy and two others fall into waters in a Sioux Falls park . The boy is rescued , but the two others -- one of them his sister -- are n't . Authorities find the sister 's body Friday ; they 're still looking for the man . `` Everybody here is essentially eyes on the water , '' a fire chief says of the search .