Google has been secretly collecting and analyzing detailed personal health information on millions of Americans spanning 21 states, according to the Wall Street Journal.

The effort, code-named "Project Nightingale," appears to be the largest successful effort by tech giants - including Amazon, Apple and Microsoft - to gain access to personal health data in order to make inroads in the healthcare industry.

Google began the effort last year with St. Louis-based Ascension, the second-largest health system in the U.S., with the data sharing accelerating over this summer and fall, the documents show. The data involved in Project Nightingale pertains to lab results, doctor diagnoses and hospitalization records, among other categories, and amounts to a complete health history, including patient names and dates of birth. -Wall Street Journal

Neither doctors nor patients were notified of the program, while at least 150 Google employees have access to much of the data according to the report, citing a personal familiar with the program and documents.

Employees at Ascension have sounded ethical and technical alarms over the way the data is being harvested and shared, however privacy experts say the data collection may be allowed under the Health Insurance Portability and Accountability Act of 1996 (HIPA), which allows hospitals to share patient data with business partners as long as it's only used "to help the covered entity carry out its health care functions."

With Project Nightingale, Google is using the data to design AI-powered software that uses machine learning to custom tailor care to individuals.

Staffers across Alphabet Inc., Google’s parent, have access to the patient information, documents show, including some employees of Google Brain, a research science division credited with some of the company’s biggest breakthroughs. A Google spokeswoman said the project is fully compliant with federal health law and includes robust protections for patient data. An Ascension spokesman had no immediate comment.

The project is currently housed under Google's cloud division, and is not charging for services in the hopes of creating an omnibus search tool for centrally accessed patient data, according to documents.

Ascension is a Catholic chain of 2,600 hospitals, doctors' offices and other facilities, which aims to improve patient care - as well as "mine data to order up more tests or determine where it might be able to make more money from an individual patient."