Baltimore’s chief prosecutor announced charges Friday against six police officers — including four charged with second-degree murder, manslaughter or involuntary manslaughter — in the death of Freddie Gray last month. Maryland State’s Attorney Marilyn Mosby said at a press conference that Gray was arrested without probable cause; transported in a police van handcuffed, shackled and unrestrained, which caused him serious injury; and not given prompt medical attention. He died a week later.

It’s very rare for police officers anywhere in the country to be charged in connection with homicide, and even rarer for them to be convicted. But officers in the Baltimore Police Department are more likely than average to be charged with crimes of all kinds, most of them far less serious: They were charged at twice the average rate of officers in other big-city forces in a recent seven-year period.

Very few of the 1,000 or more incidents in which people are killed by police officers each year lead to charges against the officers, which makes Mosby’s announcement Friday an outlier. Reports from Talking Points Memo, The Wall Street Journal and The Washington Post suggest about four or five police officers each year are charged criminally for on-duty fatal shootings.

Each of those media reports relied on data collected by Philip M. Stinson, a Bowling Green State University criminologist, using Google Alerts for media coverage. Stinson has said his method probably doesn’t miss many charges in police killings because those usually are reported by local media.

Gray wasn’t shot, so his case wouldn’t have been counted in those reports. The number of police officers charged with killings of all kinds, not just fatal shootings, is higher — 126 cases with the most serious charge of murder and nonnegligent manslaughter between 2005 and 2011 (two in Baltimore), according to Stinson, and 64 more cases where the most serious charge was negligent manslaughter (none in Baltimore).

But the Washington Post analysis shows that Gray’s case may share certain attributes that often are present — and perhaps necessary — in the rare cases when police officers are charged in connection with a killing. Part of Gray’s arrest was caught on camera. Gray would probably also be classified as unarmed, since according to Mosby he was restrained when he suffered the fatal injuries. According to Stinson’s data, police officers are more likely to be convicted when they kill someone without a firearm. His database includes 48 arrests of police officers in connection with on-duty killings not involving a firearm. In the 41 of those cases for which he has conviction data, 25 — 61 percent — resulted in conviction.

None of the data that could put Gray’s death into context is collected officially: not how many people are killed by police officers, nor how many police officers are charged in connection with homicides or with other crimes. Unofficial efforts, using media reports, fill the gap. They typically count about 1,000 people killed each year, which might miss about 200 or 300 cases that aren’t reported in the media. From the start of 2014 through March 2015 — before Gray’s death — seven people in Baltimore were killed by police, according to the data-collection project Mapping Police Violence. That’s above the average per-capita rate for the country’s 100 biggest cities.

Stinson’s data set, funded by a National Institute of Justice grant, also covers arrests and criminal charges of police officers. He and his collaborators have coded the data from 2005 to 2011, and found that Baltimore Police Department officers were arrested at a rate that is about double the rate of departments in the neighboring big cities of Philadelphia and Washington, D.C., and more than double the rate among the 200 biggest law-enforcement agencies nationally. That could reflect a higher rate of misconduct among Baltimore police officers, or a higher level of willingness to prosecute officers — or both. The Baltimore Police Department didn’t respond to my request for comment Tuesday on the above-average rate of people killed by police in its city, and the above-average rate of officers arrested from the department.

A criminal charge is a long way from conviction and imprisonment. And police officers charged with crimes are less likely than other people to be convicted and incarcerated. Among officers charged nationally in 2010 with misconduct — a broad category that includes excessive force (including fatal excessive force), sexual misconduct and assault — 33 percent were convicted and 12 percent were incarcerated. The equivalent percentages for felony defendants in the general population in 2006 were 68 percent and 48 percent, respectively. In Baltimore, though, the rate of conviction has been higher. Stinson has conviction data on 46 cases involving police officers charged with crimes between 2005 and 2011. Nearly three in four of those cases — 34 in all — resulted in conviction.