HONOLULU (HawaiiNewsNow) - A mysterious oily substance has been reported in the waters off Ala Moana Beach Park. About a dozen beachgoers have complained to lifeguards about dark spots and streaks showing up on beach toys and surfboards. The U.S. Coast Guard began testing samples earlier this week, but were not able to find any evidence of an oil spill during overflights on Tuesday. Officials said photographs of the substance did appear to resemble what's known as a tarball, a dark-colored piece of oil. Samples have been sent to a Coast Guard lab in Connecticut for further analysis.