(CNN) -- The Seattle Police Department has used weapons either excessively or unnecessarily more than half the time during arrests, the U.S. Justice Department announced Friday.
The finding was the result of a lengthy investigation that concluded the Seattle department engaged in a pattern of excessive force in violation of the Constitution and federal law.
The investigation, which began in March, was conducted by the U.S. attorney's office in Washington state along with the Justice Department's Civil Rights Division.
"The problems within SPD have been present for many years and will take time to fix," said Thomas Perez, assistant attorney general for the Civil Rights Division.
The Justice Department said it reviewed thousands of pages of documents, including training materials and written policies, video footage, internal reports and investigative files. Hundreds of interviews were conducted with officers, supervisors, city officials, community members and local advocates.
The Justice Department also looked at whether the Seattle Police Department used discriminatory policing. According to the Justice Department, it did not make a finding that officers engaged in a pattern of discriminatory policing, but "the investigation raised serious concerns that some of SPD's policies and practices, particularly those related to pedestrian encounters, could result in unlawful discriminatory policing."
Investigators looked at a random sample of force reports between 2009 and 2011 and concluded when Seattle Police Department officers use force, they did so in an unconstitutional manner nearly 20% of the time. It found officers too quickly resorted to using impact weapons such as batons and flashlights.
"SPD officers escalate situations, and use unnecessary or excessive force, when arresting individuals for minor offenses," the Justice Department said. "This trend is pronounced in encounters with persons with mental illnesses or those under the influence of alcohol or drugs."
Justice officials found that problems within the police department were exacerbated by the large number of less-experienced officers. Approximately one-third of the officers had three years or less experience, and more were likely to be hired, as another 350 officers were retirement eligible.
"Proper leadership, training (including mentoring), and oversight are critical for molding this next generation of SPD officers," said Justice officials in a letter sent to Seattle Mayor Michael McGinn and Police Chief John Diaz.
In an effort to restore public confidence and trust, the Seattle Police Department has begun to implement a number of remedial measures, including new force policies and training for officers on how to conduct effective and constitutional policing, according to the Justice Department.
"We remain optimistic about the changes that are to come. Failure is not an option," Perez said.