= Solution
The administrative data include only encounters with $M=1$. Conditioning on the stop indicator selects on the <collider>
$$
D\to M\leftarrow U,
$$
which opens the noncausal path $D\leftrightarrow U\to Y$ and creates <collider bias>. The equal entries estimate only the selected risks $\mathbb P(Y=1\mid D=d,M=1)$. They ignore racial differences in the probability of being stopped and therefore do not identify the total <causal effect> of race on violence. This is also <selection bias>, because encounters with $M=0$ never enter the dataset.
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