PRESSURED INTO SILENCE PRESSURED INTO SILENCE Ways police departments say witnesses are pressured: Implicit threats, looks or gestures: 80% Explicit threats of violence: 63% Actual physical violence: 53% Property damage: 45% Other: 14% Note: 71% indicated more than one Source: Police Executive Research Forum Survey Heather Fong By Robert Galbraith, Reuters WASHINGTON  Witnesses to killings and other violent crimes refuse to cooperate in law enforcement investigations with such regularity that their silence is driving down the rate of solved murders throughout the country, police officials say. In a survey of 76 police agencies released this month by the Police Executive Research Forum, a national police advocacy group, 78% cited a decreased willingness to testify among witnesses, and 45% reported a drop in the rate of solved cases. Trenton, N.J., Police Director Joseph Santiago says the lack of cooperation extends to child murders and other crimes that once generated public outrage. "We're not getting information from the average person anymore," Santiago says. Even as violent crime plunged to historic lows during the past decade, authorities say the murder clearance rate also dropped — from a high of 69% in 1998 to 60% in 2006, the last full year measured by the FBI. A case is cleared when there is an arrest, the suspect is charged and the case is referred for prosecution. FBI spokesman John Miller says criminals have successfully "marketed a climate of fear" in some places to silence potential witnesses — "a powerful force and not one that some police executives are trained to deal with," Miller says. San Francisco Police Chief Heather Fong says the phenomenon is not confined to any one region. Cities in which police cite persistent problems with witness cooperation: • In Minneapolis, Police Chief Tim Dolan says investigators encounter reluctant witnesses in 30% of murder investigations and more than 50% of other violent crime inquiries. Dolan says the clearance decline is due in part to "codes of silence" that are increasingly common in cities where gangs use threats of retaliation to block law enforcement. • In Washington, incidents of witness intimidation were up 45% last year, says Cynthia Wright, chief of the local U.S. attorney's Victim-Witness Assistance Unit. • In Boston, Police Commissioner Ed Davis says witnesses' fear of retaliation was directly related to low murder clearance rates that hovered near 38% in 2006. To bolster public cooperation, Davis says the department installed an anonymous text-messaging system last year. It strips off the identity of the senders, who provide initial leads in building criminal cases. Within the past two years, Davis doubled the number of investigators and supervisors dispatched to homicide scenes, allowing for more contacts with potential witnesses. Since 2006, the clearance rate has inched up to 40%, and authorities hope to push it over 50% by this year. "The more certain punishment" for the suspect, "the more certain people will be about coming forward," Davis said. Conversation guidelines: USA TODAY welcomes your thoughts, stories and information related to this article. Please stay on topic and be respectful of others. Keep the conversation appropriate for interested readers across the map.