Authorities last year suggested Chicago’s spike in gun violence in 2015 was due to the “Ferguson effect” — cops afraid to do their jobs because of the scrutiny following the shooting of a black teenager by a white officer in 2014 in Ferguson, Missouri.

But with Chicago now plagued with an even steeper rise in fatal shootings in January, compared with the same period of 2015, street cops are offering a new reason: the “ACLU effect.”

They say the Chicago Police Department’s pact with the American Civil Liberties Union of Illinois to monitor police stops in greater detail is prompting officers to stop policing, leaving the streets to the criminals and leading to the spike in gun violence.

Starting in January, officers have been required to fill out two-page forms documenting every stop of a citizen for everything from traffic violations to investigative stops. They ask for much more information than the previous one-page “contact cards” officers filled out.

In interviews with officers and sergeants over the past month, a common theme has emerged: Cops say they have avoided making many of the stops they would have routinely done last year. They fear getting in trouble for stops later deemed to be illegal and say the new cards take too much time to complete.

Their reluctance to make stops was borne out by a police statistic released Sunday: Officers completed 79 percent fewer contact cards in January 2016 than over the same period last year.

Meantime, January 2016 was the deadliest first month of the year since 2001, according to a Chicago Sun-Times review of homicide records. At least 50 murders were recorded through 6 a.m. Sunday — most the result of shootings on the West and South Sides.

In comparison, there were 28 murders in January 2015 and 20 in 2014. More than 270 people were victims of gun violence this January, including those who died, compared with 133 in January 2015.

The “unacceptable increase in violence was driven primarily by gang conflicts and retaliatory violence,” Interim Police Supt. John Escalante said.

“The vast majority of incidents originated from petty disagreements that escalated into gun violence that tore apart families,” Escalante said. “Chicagoans should know that detectives are making progress in January’s investigations and have already solved 14 murder cases this month.

Eight of those cases occurred in 2016, with the rest from previous years.

To combat the rising violence, the department has redeployed more than 350 police officers and 31 sergeants from foot patrol into vehicles to give them greater visibility and improve their ability to respond to violent incidents.

And to address the sharp drop in officers producing contact cards — a bellwether of police activity — Escalante said: “We are conducting training on the new investigative stop law and reporting requirements and that is taking place three shifts per day.”

One sergeant in a South Side police district attributed the latest wave of violence to criminals becoming emboldened by what they see as officers being hamstrung by the intense scrutiny over police practices since the video of an officer fatally shooting Laquan McDonald was made public in November.

The officer, Jason Van Dyke, faces murder charges in the 2014 shooting of McDonald, a 17-year-old who was holding a knife but appeared to be walking away from the officer when he was shot.

The sergeant said officers worry that the legality of their own street stops will come under question in the current environment of public scrutiny over police practices. Under the department’s deal with the ACLU, a retired federal judge will review the contact cards for any potential constitutional violations and issue a report in June.

The sergeant said his team made dozens of stops in December, but only a handful in January.

“We’re avoiding all the gray areas,” he said.

But the ACLU rejects any correlation between declining street stops and rising violence, said Karen Sheley, director of police practices for the ACLU of Illinois. Other cities have scaled back their street stops without an explosion of shootings, she said.

The reduction of “invasive” street stops is actually a good thing, Sheley said.

The ACLU had pushed for the changes to allow for better monitoring of stop-and-frisk practices and their impact on minorities. The ACLU released a report in March that found blacks accounted for 72 percent of stops between May and August of 2014, but just 32 percent of the city’s population.

The ACLU also reviewed 250 contact cards filled out after stops, and found that half didn’t list a lawful reason for them.

Under similar pressure from the ACLU, New York City has scaled back its stop-and-frisk practices. New York police made 23 stops per 1,000 people in 2011 and 2 per 1,000 in 2014, compared with 93 per 1,000 in Chicago in the summer of 2014.

Through Jan. 24, New York City has seen a 26 percent decrease in murders compared with the same period of 2015, according to its police department.

Contributing: Sam Charles