The King County Sheriff's Office is trying to find out who put ball bearings into the new South Lake Union streetcar tracks on Wednesday...

The King County Sheriff’s Office is trying to find out who put ball bearings into the new South Lake Union streetcar tracks on Wednesday afternoon, causing a brief train delay.

The incident is being investigated as reckless endangerment, a gross misdemeanor, sheriff’s spokesman Sgt. John Urquhart said Thursday.

One of the bearings, shown to The Seattle Times by a transit supervisor Wednesday night, was a bit larger than a golf ball and weighed more than 1 pound.

A train was delayed 10 minutes for track inspections, after it hit one of the bearings on Terry Avenue North, around 5 p.m. That bearing was wedged into a groove in the trackway, and caused a jolt aboard the streetcar, said Pat Daniels, a Metro Transit operations supervisor.

Deputies have recovered one ball, and a case report says four others were found. Deputies will obtain those from the transit employees who picked them up, Urquhart said.

Wednesday was opening day for the streetcar line, which stretches from the Westin Hotel to the Fred Hutchinson Cancer Research Center.

No train damage was reported. Damaging a train is a felony, said Urquhart. Another risk is that a bearing could shoot out and hit someone, he said. The risk of a derailment from a ball bearing in this situation is “minimal,” said Tom Irion, streetcar startup manager for Metro.

Streetcar tracks are inspected at least daily to remove any debris in the rails or switches.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com