The National Highway Traffic Safety Administration's Mark Rosekind criticised FCA's recall of 11 million cars and handling of fatal accidents.
He told a hearing in Washington of failures to inform owners of recalls, delays in repairs, and not doing work.
Fiat said it "learned from [its] mistakes and missteps".
The recalls cover Jeep, Dodge, Chrysler and Mitsubishi vehicles from model years dating back to 1993.
Much of the testimony in the hearing focused on a recall of 1.56 million older-model Jeeps with gas tanks mounted behind the rear axles.
The tanks are vulnerable to being punctured in a rear crash. At least 75 people have died in fires involving the Jeeps, according to the NHTSA.
"There will be action," Mr Rosekind said. "What you've heard here is that there's a pattern that's been going on for some time."
He did not detail the sanctions, although they could be a mixture of fines and orders to buy back or replace vehicles.
The NHTSA has yet to decide whether to forward the case to the Justice Department for possible criminal action.
"Recall execution is where we have fallen short," said Scott Kunselman, FCA's senior vice president for vehicle safety and regulatory compliance in North America. "We have learned from our mistakes."
Fiat-Chrysler Automobiles (FCA) faces action over the mishandling in the US of car recalls, the country's top vehicle safety regulator has said.