* ION has acquired prominent role in Italy's financial sector

* Central bank looking into sprawling corporate structure

* ION's growth fuelled by debt

ROME, Jan 25 (Reuters) - A string of acquisitions by Andrea Pignataro's fintech firm ION in Italy's financial sector has prompted the central bank to study in detail the group's complex ownership structure, two sources with knowledge of the matter told Reuters on Thursday.

The move highlights the growing scrutiny drawn by a group that has invested 6 billion euros ($6.5 billion) in Italy in recent years, gaining a prominent role in the financial sector as a supplier of key services, and control of sensitive data.

The Bank of Italy declined to comment. Representatives for ION were not immediately available to comment.

Supervisory powers over the country's financial sector also allow the Bank of Italy to look into companies that own banking and financial businesses, and the sources said the central bank has delved into ION's intricate corporate structure.

Its role as a supplier of services essential to the financial industry, such as trading and clearing, came into prominence a year ago when the group suffered a ransomware attack that left scores of brokers unable to process derivatives trades.

The Bank of Italy's focus on ION relates also to the huge amount of sensitive financial data the group manages following its acquisitions, the sources said.

While according to one of the people the investigation has been ongoing for some time, the other said requests to the group to disclose information had intensified in the last few months of 2023.

Hailing from the northern town of Bologna, Pignataro made his fortune abroad building a group with a corporate architecture spanning several countries which provides financial software systems and IT infrastructure, as well as data and analytics.

In Pignataro's first major foray into his native Italy, ION in 2021 acquired Cedacri, Italy's main provider of outsourced IT services to banks. Later that year, ION bought credit data group Cerved.

It also holds stakes in banks Monte dei Paschi di Siena , illimity and Cassa di Risparmio di Volterra.

This month ION secured a green light from Italy's government to acquire real estate and bad loan manager Prelios, after Rome vetted the deal under legislation designed to protect assets of strategic national relevance.

However, sources told Reuters Italy intends to launch a further inquiry when ION pledges Prelios shares as collateral against debt covering almost half of the 1.35 billion euro acquisition.

ION is known in the industry for its aggressive use of financing in its acquisition strategy.

A publicity-shy character with a knack for numbers, Pignataro has not disclosed his longer-term plans. Critics say ambitions to build a financial data and services conglomerate in Italy are a bet on a mature market with few growth prospects.

($1 = 0.9212 euros)

(Editing by Jan Harvey)