Index locking?

After getting complaints about slow process we found out that one user's session was blocking another user's session. I found out that the waiter's session was waiting to acquire a lock on an index. This is based on data from v$session of the waiting session.

The index was built/rebuilt about 10 months ago - it could not have been index rebuild that could have caused the lock. I tried googling to find more information about locking of indexes but not able to find anything that can help me. ver 10.2. Can anyone explain or point to information on index locking.

Thanks.

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