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We report on 230 GHz (1.3 mm) VLBI observations of M87 with the Event HorizonTelescope using antennas on Mauna Kea in Hawaii, Mt. Graham in Arizona andCedar Flat in California. For the first time, we have acquired 230 GHz VLBIinterferometric phase information on M87 through measurement of closure phaseon the triangle of long baselines. Most of the measured closure phases areconsistent with 0$^{\circ}$ as expected by physically-motivated models for 230GHz structure such as jet models and accretion disk models. The brightnesstemperature of the event-horizon-scale structure is $\sim 1 \times 10^{10}$ Kderived from the compact flux density of $\sim 1$ Jy and the angular size of$\sim 40 $ $\rm \mu$as $\sim$ 5.5 $R_{{\rm s}}$, which is broadly consistentwith the peak brightness of the radio cores at 1-86 GHz located within $\sim10^2$ $R_{{\rm s}}$. Our observations occurred in the middle of an enhancementin very-high-energy (VHE) $\rm \gamma$-ray flux, presumably originating in thevicinity of the central black hole. Our measurements, combined with results ofmulti-wavelength observations, favor a scenario in which the VHE region has anextended size of $\sim$20-60 $R_{{\rm s}}$.

230 GHz VLBI observations of M87: event-horizon-scale structure at the enhanced very-high-energy $\rm \gamma$-ray state in 2012