The authorities have been turning hostile and preventing people from leaving the city, firing shots, spreading misinformation, and verbally humiliating groups trying to leave the city on foot.
Read about it from a first-hand account here.

Unfortunately, our sinking feeling (along with the sinking City) was correct. Just as dusk set in, a Gretna Sheriff showed up, jumped out of his patrol vehicle, aimed his gun at our faces, screaming, "Get off the fucking freeway". A helicopter arrived and used the wind from its blades to blow away our flimsy structures. As we retreated, the sheriff loaded up his truck with our food and water.
Once again, at gunpoint, we were forced off the freeway. All the law enforcement agencies appeared threatened when we congregated or congealed into groups of 20 or more. In every congregation of "victims" they saw "mob" or "riot". We felt safety in numbers. Our "we must stay together" was impossible because the agencies would force us into small atomized groups.
In the pandemonium of having our camp raided and destroyed, we scattered once again. Reduced to a small group of 8 people, in the dark, we sought refuge in an abandoned school bus, under the freeway on Cilo Street. We were hiding from possible criminal elements but equally and definitely, we were hiding from the police and sheriffs with their martial law, curfew and shoot-to-kill policies.
...
Throughout, the official relief effort was callous, inept, and racist.