AMT (Aluminium Model Toys) was established in Troy, Michigan in 1948. At first, they made plastic promo models but later moved to metal kits. These became very popular and are highly collectible today. Most of the model cars were American trucks and cars in 1/25 scale. Lster, AMT also produced Hot rods, customs, and other vehicles.
In 1978, AMT was bought by British Lesney, makers of Matchbox diecast vehicles. Lesney moved the firm to Baltimore, and closed the Maple Road facility in Troy.
Prices of plastics had increased by this time, and Detroit was squeezed by government regulations of safety, emissions and fuel economy. Detroit sponsored fewer and fewer promotionals so model companies depended more on kits. The model building hobby declined as well.
In 1983, Ertl took over AMT and formed AMT-Ertl. Many old truck kits were reissued, but new models were rare.
Many of the vintage AMT model kits can be found on auction sites, often for fairly high prices.
See for more info and history this AMT article on Wikipedia.