Citizen complaints against Denver police officers spiked this summer — up 38 percent from a year ago — for reasons officials struggled on Monday to pinpoint.

But an activist group said the increase is likely a direct result of its efforts to encourage the community to report problems with police.

Members of the public filed 196 complaints in July, August and September, compared with 142 during the same period in 2011, according to a quarterly report compiled by Nicholas Mitchell, the city’s independent monitor.

That number is also higher than it was during the second quarter of 2012, when citizens lodged 102 complaints against police, ranging from inappropriate force and biased policing to harassment and improper procedure.

Discourtesy, though, has remained the public’s top accusation over time.

“It’s notoriously difficult to estimate why complaint numbers rose and why they fall,” Mitchell said. He will hire a second research analyst this year whose duties will include studying complaints both topically and geographically in search of trends.

The numbers were included in Mitchell’s first report to the public since he was appointed to the watchdog post in August.

The quarterly report also tracks the number of allegations made by citizens against Denver officers; a single complaint could spawn multiple allegations. There were 92 allegations of discourtesy by Denver officers in the third quarter of 2012, compared with 59 such accusations in the second quarter. Citizens said officers were discourteous 70 times during the third quarter of 2011.

Allegations concerning “responsibilities to serve the public” also rose from 46 in the second quarter to 78 in the third, the report says. There were just 28 such citizen complaints in the first part of the year. Citizens said officers used inappropriate force 39 times in the third quarter of 2012 compared with 32 times during the same period in 2011.

“This is really a response by our community,” said Mu Son Chi of the Colorado Progressive Coalition, whose members went door-to-door in some Denver neighborhoods this summer to find out if residents had negative experiences with officers and whether they had reported them. Many had bad experiences, he said, but fewer knew how to report them. The group taught them where and how.

“I’d say we played a significant role in that increase,” Chi said.

Citizen complaints filed to Mitchell’s office are forwarded to the department’s Internal Affairs Bureau for review, which could take weeks or months. It is unclear how many of the complaints filed this summer have been sustained.

The number of complaints against officers filed by others in the department, meanwhile, has stayed relatively unchanged at about 24 each quarter.

The increase in citizen complaints is not necessarily a bad sign, Denver Police Department spokesman Sonny Jackson said. It could mean residents have recently felt more comfortable airing their concerns. Police Chief Robert White “has been very much out there and shown his willingness to listen,” Jackson said. “They feel like they can reach out to us and we’ll listen.”

He also noted that the numbers might not be indicative of any trend because complaints might have been filed stemming from incidents that happened months earlier.

“A lot of times it’s just certain things that ebb and flow,” Jackson said. “Why things cycle the way they do is hard to say.”

Still, he added, departmental training aims to teach courteousness and ways of interacting with the public.

But Nick Rogers, president of the Denver Police Protective Association, was dismissive of Mitchell’s report, saying he had heard of no such problems from the rank and file.

“I don’t put a lot of stock in that report,” Rogers said. “I just don’t give it a lot of credibility.”

Sadie Gurman: 303-954-1661, sgurman@denverpost.com or twitter.com/sgurman