The AT&T building had hazardous materials in it
The blue 3 is the health classification
Risk level 3: Materials extremely hazardous to health, but areas may be entered with extreme care. Full protective clothing including self-contained breathing apparatus, coat, pants, gloves, and boots, with bands around the legs, arms, and waist should be provided. No skin surface should be exposed.
The red 4 is the fire risk classification
Flash point of whatever was being stored there was less than 73F
Classification doc here
https://www.atsdr.cdc.gov/MHMI/mhmi-v2-a.pdf