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."

In a nod to Mr Trudeau's fondness for statement socks, he wore Canada-themed mountie and maple leaf print ones to welcome his first foreign visitor.
Mr Trudeau opted for a more subtle striped pair on his first official engagement in the Republic of Ireland.
The Canadian premier is in the country for a three-day visit.
Away from talk of socks, trade issues are set to dominate the agenda, with the Canada-EU free trade deal expected to be a focal point of talks between the two leaders.
Mr Trudeau arrived in Dublin with his wife, Sophie GrÃ©goire, and their youngest son, Hadrien, on Monday.
He is in Europe for a key G20 summit in Hamburg, Germany later this week.
Mr Trudeau is also due to meet Irish President Michael D Higgins at his official residence.
The visit was arranged in May when Mr Varadkar's predecessor Enda Kenny met the Canadian leader in Montreal.
On that occasion, Mr Trudeau wore special Star Wars themed socks, as it coincided with the widely-celebrated Star Wars Day on 4 May.

GB will play Australia, ranked second in the world, in Saturday's final at 13:35 BST.
Sam Ward gave Britain the lead from a penalty corner before Dominic Newman levelled for the Black Sticks.
But strikes from Phil Roper and Mark Gleghorne carried home Bobby Crutchley's side.
Britain's return to the final for the first time since England lifted the trophy in 1994 was confirmed when hosts Malaysia beat India 1-0 later in the day.
"It was a very good performance in stifling heat. I felt we dominated the game," said Crutchley.
"If we had a better conversion rate from the chances we created, the scoreline would have been more comfortable."
Results, fixtures & tables on the tournament's official website
Fiat-Chrysler Automobiles (FCA) faces action over the mishandling in the US of car recalls, the country's top vehicle safety regulator has said.
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Irish Taoiseach (PM) Leo Varadkar has engaged in some "sock diplomacy" in his first meeting with Canadian Prime Minister Justin Trudeau in Dublin.
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Great Britain claimed a 3-2 win against New Zealand in the Sultan Azlan Shah Cup to become the first British side to reach the final in 23 years.