Sagan, racing with number one as defending champion, fell with Oliver Naesen (Ag2r La Mondiale) and Greg Van Avermaet (BMC Racing).
They were racing with 16.9km until the finish in Oudenaarde, at 59 seconds behind solo leader and eventual winner Philippe Gilbert (Quick Step Floors).
"I think you can see from the video what happened. It was my fault, but I just don't know," Sagan explained.
"I was near the barrier, yeah, but I was controlling the space; how close I was. I think it was a pullover or something, I didn't know.
"If you hit a barrier, you are right on the ground, your bike remains there, but I felt something and I was able to continue ahead.
"I didn't fall right away, and my bars didn't spin, then something gave me a problem and right away the other riders hit me from behind and I wasn't able to continue."
Sagan spoke in Italian as he rode his bike from anti-doping to the hospital next door. He said that he can normally ride on the left side at the top of the Oude Kwaremont, where the cobbles continue next to the massive white VIP tents, without problems.
"Every year, if you are doing it alone, you can do it. I was alone ahead and they were following me, but then I snagged on a pullover or I don't know, some flag. Then the barrier was after, I went towards the barrier. Beforehand, I was far enough from the barrier, but I snagged something"
Asked if he hurt, Sagan pointed down to his right hip. He said, "la anca." He went inside for X-rays and other checks.
Sagan's suffering began when the race split on the Muur with 95 kilometres to race. Quick-Step rode away under Tom Boonen's power. It had three men in a 14-man move that included Sky’s Luke Rowe.
"There were various crashes on the Muur. I couldn't get up to the front. I thought that Quick-Step would do something there, but the others crashed two times, and I was a little bit behind, just that it was all blocked with riders everywhere," Sagan added.
"I was not far off after the Muur, but Trek pulled hard to catch me. So thought, 'OK, if they are pulling and take me, and they continue to pull to get the escape,' but they stopped pedalling when they caught me.
"We were at six seconds and I don't know why they didn't' take them back. That was a little mistaken because we so near to catching them and they didn't do it.
"That's the thing that, yet again, they raced as if they were OK and I was just there alone, but the escape was up the road and someone from that escape won."
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