(Reuters) -UnitedHealth Group ( UNH ) CEO Andrew Witty has stepped down unexpectedly and company veteran Stephen Hemsley is returning to the top role to steer the healthcare conglomerate through one of its most challenging periods.

The company has grappled with a series of challenges over the past year: a surge in medical costs, a cyber attack at its tech unit, the killing of its insurance unit head and subsequent public backlash against the industry.

The following is a timeline of key events for the company since 2024:

January 12, 2024

UnitedHealth ( UNH ) shares fell after medical services costs at the healthcare conglomerate surpassed Wall Street expectations for the first time in two years, even as it beat fourth-quarter profit and revenue estimates.

February 21, 2024

UnitedHealth Group's ( UNH ) tech unit, Change Healthcare, is hit by a cybersecurity attack on its systems.

February 28, 2024

In a message posted to, and then quickly deleted from their darknet site, the hackers blamed for the strike say they stole millions of sensitive records, including medical insurance and health data, from the company.

February 29, 2024

UnitedHealth Group ( UNH ) says the cyber attack was perpetrated by hackers who identified themselves as the "Blackcat" ransomware group.

March 13, 2024

The U.S. government says it has opened an investigation into the cyber attack to find out whether there was a breach of protected health data and if the company followed U.S. health privacy law.

March 13, 2024

UnitedHealth Group ( UNH ) says its unit Change Healthcare's pharmacy network is back online, weeks after the fallout from the cyber attack swept across the country's healthcare system that depends heavily on insurance.

April 16, 2024

UnitedHealth Group ( UNH ) says it expects the hack of its Change Healthcare unit to cost the company up to $1.6 billion in 2024, but maintained its annual earnings forecast, suggesting the cyber attack's impact was less severe than many on Wall Street had feared.

May 1, 2024

Hackers who breached UnitedHealth's ( UNH ) tech unit in February potentially stole a third of Americans' data, the largest U.S. health insurer's CEO tells a Congressional committee.

July 16, 2024

UnitedHealth ( UNH ) forecasts a bigger hit to annual earnings from the February hack at its tech unit, but maintains its full-year profit forecast.

September 20, 2024

The U.S. Federal Trade Commission sues the country's three largest pharmacy benefit managers, accusing them of steering diabetes patients towards higher-priced insulin to reap millions of dollars in rebates from pharmaceutical companies.

October 15, 2024

UnitedHealth Group ( UNH ) provides a 2025 profit forecast below Wall Street estimates as it expects pressure across its government-supported health insurance businesses.

November 12, 2024

The U.S. Department of Justice and three U.S. states file a lawsuit to block UnitedHealth Group's ( UNH ) $3.3 billion acquisition of Amedisys, citing concerns that the deal would reduce competition in the home health services market.

December 4, 2024

Brian Thompson, the CEO of UnitedHealth's ( UNH ) insurance unit, is killed outside a Midtown Manhattan hotel ahead of the company's investor day in what police described as a targeted attack by a gunman lying in wait for him.

December 19, 2024

The suspect in the killing of Thompson is charged with federal murder and stalking crimes in a Manhattan courtroom, alongside state murder and terrorism charges previously announced by New York prosecutors.

December 23, 2024

Luigi Mangione, the man accused of fatally gunning down Thompson on a Manhattan street, pleads not guilty to New York state murder charges that brand him a terrorist.

January 16, 2025

At the company's first earnings call since the murder of Thompson, CEO Andrew Witty says healthcare in the U.S. needs to be "less confusing, less complex and less costly."

Witty says the company will work with policymakers to reduce the frequency of prior authorization approvals required before a patient can access medical treatment in its Medicare business for people aged 65 and older or with disabilities.

January 23, 2025

UnitedHealth Group ( UNH ) names Tim Noel, the head of its Medicare business, to the top job at its health insurance business, a month after the unit's former CEO Brian Thompson was killed.

January 24, 2025

The company says the cyber attack at its tech unit last year affected the personal information of 190 million people, making it the largest healthcare data breach in the United States.

February 19, 2025

UnitedHealth's ( UNH ) insurance unit offers employees in its benefits operations unit the option to accept buyouts if they quit by March 3, CNBC reports.

February 21, 2025

The U.S. Justice Department is investigating UnitedHealth's ( UNH ) Medicare billing practices, the Wall Street Journal reports. The health insurer says it is unaware of any new probe underway.

February 25, 2025

U.S. Senator Chuck Grassley requests detailed information on UnitedHealth's ( UNH ) Medicare billing practices in a letter to CEO Andrew Witty.

April 4, 2025

The U.S. Federal Trade Commission's lawsuit against pharmacy benefit managers over insulin pricing practices will likely move forward again after being paused amid President Donald Trump's firing of the agency's two Democratic commissioners.

April 17, 2025

UnitedHealth Group ( UNH ) surprises investors with what its CEO said was an "unusual and unacceptable" quarterly earnings miss, and lowers its outlook for the full year due to higher-than-expected medical costs, sparking a more than 20% selloff in shares that reverberated across the sector.

April 21, 2025

UnitedHealth Group ( UNH ) says it spent nearly $1.7 million on security for its top executives in 2024, months after the fatal shooting of senior executive Brian Thompson.

May 7, 2025

The company is sued for allegedly concealing how backlash from the killing of Thompson was damaging its business.

May 13, 2025

UnitedHealth Group ( UNH ) suspends its 2025 forecast due to surging medical costs, and CEO Andrew Witty steps down suddenly.

(Reporting by Puyaan Singh and Sneha S K in Bengaluru; Editing by Devika Syamnath)