Catch the Explorer

One evening, four explorers named Xavier, Yeoman, Zenger, and Osborn made separate camps along the banks of a river.

Wilson, a fifth explorer camped in the country’s interior, communicated with the other men by radio at various times during the night. When he received no reply from Osborn after 10:30 that night, Wilson communicated with the other three men to express his concern.

The next morning Osborn was found dead; he had been murdered.

Evidence at the scene of the crime indicated that the killer had approached Osborn’s camp by boat from the river. Xavier, Yeoman, and Zenger had each had access to a canoe on the previous night.

Wilson suspected that either Xavier, Yeoman, or Zenger had killed Osborn. From the following facts, Wilson was able to eliminate two of these men as suspects.

1. Osborn was killed in his camp before 10:30 on the previous night; he had been shot and had died instantly.

2. The killer traveled to Osborn’s camp and returned to his own camp by canoe.

3. Xavier’s camp was located directly downstream from Osborn’s camp, Yeoman’s camp was located directly across the river from Osborn’s camp, and Zenger’s camp was located directly upstream from Osborn’s camp.

4. At least 80 minutes were needed for each of the three men to make the round trip to and from Osborn’s camp by canoe.

5. There was a strong current in the river.

6. Wilson received replies to his radio calls at the following times:

From Osborn – 9:15
From Xavier – 8:15, 9:40, and 10:55
From Yeoman – 8:20, 9:45, and 11:00
From Zenger – 8:25, 9:50, and 11:05

Which one of the three men could Wilson not eliminate as a suspect?