Police said they responded just before 11 a.m. Friday to the area of Highland Avenue and State Street for a report of a male suspect interfering with railroad operations.
The suspect ran from police and jumped into the Fox River, police said.
Police saw he had a large knife.
Elgin fire crews responded in an effort to get the suspect out of the water.
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At some point, the suspect went under the water, police said.
More resources were called to the area to search for the suspect, police said.
Chopper 7 was over the scene about noon.
Emergency vehicles could be seen, as well as a boat in the Fox River.
This is a developing story; check back for updates.