Criminal Defense

Forensic Science: Understanding The Combined DNA Index System (CODIS)

October 18, 2023 by Seppi Esfandi in Criminal Defense  Forensic Science  
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CODIS: The National DNA Database System

The Combined DNA Index System (CODIS) is a powerful tool used in the United States to store and analyze DNA profiles for criminal justice purposes. Created and maintained by the Federal Bureau of Investigation (FBI), CODIS consists of three interconnected levels: the Local DNA Index System (LDIS), the State DNA Index System (SDIS), and the National DNA Index System (NDIS). These levels allow for the sharing and comparison of DNA information at the local, state, and national levels, aiding law enforcement agencies in solving crimes, identifying missing persons, and linking related cases.

Establishment and Purpose

The idea of establishing a national DNA database in the United States was first proposed by the Technical Working Group on DNA Analysis Methods (TWGDAM) in 1989. The FBI recognized the need to maximize voluntary participation from states to avoid fragmentation in the development of DNA databases, similar to what occurred with the Automated Fingerprint Identification System (AFIS). To achieve this, the FBI provided free CODIS software to state and local crime laboratories, ensuring a unified approach to DNA databasing across the country.

In 1994, the DNA Identification Act was passed by Congress, which authorized the FBI to create a national DNA database. The act required participating laboratories to maintain accreditation from an independent nonprofit organization and ensured the proficiency and quality of the DNA profiles uploaded into CODIS. The national level of CODIS (NDIS) was implemented in October 1998, and today, all 50 states, the District of Columbia, federal law enforcement agencies, the Army Laboratory, and Puerto Rico actively participate in the sharing of DNA profiles through CODIS.

CODIS Database Structure

The CODIS database consists of several indexes, each serving a specific purpose in DNA analysis and comparison. These indexes include the Offender Index, the Arrestee Index, the Forensic Index, the Missing Persons Index, the Unidentified Human Remain Index, and more. Let’s explore some of these key indexes:

Offender Index

The Offender Index contains DNA profiles of individuals who have been convicted of certain crimes defined by state and federal laws. These profiles are obtained through the collection of DNA samples from offenders and are used to compare against DNA evidence found at crime scenes. When a match is found between an offender’s DNA profile and crime scene DNA, it can provide valuable investigative leads and help solve cases.

Forensic Index

The Forensic Index stores DNA profiles obtained from crime scene evidence, such as DNA samples collected from bloodstains, saliva, or other biological materials left behind by perpetrators. This index allows for the comparison of DNA profiles across different crime scenes, enabling the identification of patterns and connections between cases. When a match is found between two or more crime scene DNA profiles, it can indicate the work of a serial offender or link seemingly unrelated cases together.

Missing Persons Index

The Missing Persons Index is designed to assist in the identification of unidentified human remains or to help locate missing individuals. DNA profiles of missing persons or their biological relatives are stored in this index. By comparing these profiles to unidentified human remains or other missing persons profiles, CODIS can potentially provide answers and bring closure to families searching for their loved ones.

How CODIS Works

The CODIS system operates by comparing DNA profiles stored in the different indexes to identify potential matches. When a DNA profile from a crime scene (Forensic Index) matches a profile from an offender (Offender Index), it is referred to as an “offender hit.” Similarly, when two or more crime scene DNA profiles match each other, it is called a “forensic hit.” These hits are important investigative leads that can help law enforcement agencies link crimes, identify suspects, and coordinate investigations.

Upon generating a hit, laboratory personnel reanalyze the samples to validate the match. This involves performing additional tests and confirmatory analysis to ensure the accuracy and reliability of the results. Once a match is confirmed, law enforcement agencies can use this information to establish probable cause and obtain a known biological reference sample from the suspect. The suspect’s DNA sample is then compared to the crime scene evidence, providing evidence that can be presented in court.

Safeguarding DNA Information

The security and confidentiality of DNA information stored in CODIS are of utmost importance. Access to the CODIS software and the databases is limited to authorized individuals within law enforcement agencies. Physical security measures, such as secure computer terminals and servers, are in place to protect the system from unauthorized access.

Federal law, including the DNA Identification Act of 1994, ensures the confidentiality of DNA data stored in the national database. Only criminal justice agencies are allowed access to the DNA profiles for identification purposes. Personal identifying information, such as names and social security numbers, is not stored in CODIS. The DNA profiles themselves, along with associated metadata, are the primary information stored and searched within the database.

CODIS Statistics and Effectiveness

CODIS has been instrumental in aiding investigations and solving crimes since its implementation. The effectiveness of CODIS is measured by the number of investigations aided through database hits. As of September 2020, CODIS had aided in over 520,000 investigations and produced more than 530,000 hits. These hits have linked crimes, identified suspects, and provided crucial leads to law enforcement agencies across the country.

It’s important to note that each state has its own DNA database and sets its own inclusionary standards, which may vary from the national level. This means that some profiles present in state databases may not be routinely searched across state lines. However, the collaboration and sharing of DNA profiles through CODIS have significantly enhanced the investigative capabilities of law enforcement agencies.

Future Developments and Challenges

As technology advances and DNA analysis techniques improve, the potential of CODIS continues to expand. Additional loci and markers may be added to the CODIS Core, increasing the discriminatory power and accuracy of DNA profiling. Research and development efforts are ongoing to enhance the functionality and effectiveness of CODIS, ensuring that it remains a valuable tool for solving crimes and identifying individuals.

While CODIS has proven to be a powerful resource, challenges remain. The backlog of unprocessed DNA cases and samples in crime laboratories has been a persistent issue. Efforts are being made to address this backlog, but the resources required for processing and analyzing DNA samples continue to be a challenge. Additionally, ensuring the quality and accuracy of DNA profiles is crucial, as any errors or inaccuracies can have significant consequences for investigations and the justice system as a whole.


CODIS, the Combined DNA Index System, has revolutionized the field of forensic science and law enforcement by providing a centralized and interconnected database for DNA profiles. Through its three levels – NDIS, SDIS, and LDIS – CODIS enables the sharing and comparison of DNA information, aiding in the identification of suspects, the resolution of cold cases, and the reunification of missing persons with their families. As technology advances and challenges are addressed, CODIS will continue to play a vital role in the pursuit of justice and public safety.

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