Chicago police are adding "significantly more" undercover officers at the start and finish lines of the Bank of America Chicago Marathon following news that the gunman who opened fire on concertgoers in Las Vegas over the weekend had been researching Chicago in the months before the attack. 

"CPD will have heavy undercover presence at this #ChicagoMarathon," Chicago police spokesman Anthony Guglielmi tweeted Thursday. "Well over 1,000 additional officers have been added to our deployment." 

From Cubs games to the Chicago Marathon, city officials say stepped up security plans have been and will stay in place for major events happening in the city this weekend. 

As Grant Park prepares to host one of the city's largest outdoor events, the Bank of America Chicago Marathon, drawing more than 40,000 runners and hundreds of thousands of spectators, Mayor Rahm Emanuel, city law enforcement and marathon organizers have been meeting to discuss upped security plans. 

"Remember, we started changing what we were doing after the Boston Marathon... and we upped our game dramatically," Emanuel said.

Marathon organizers said Vegas remains at the top of their minds, but they're confident of the security plan they have in place and will continue to fine-tune it before Sunday.

"As we enter the final week of preparations for the 40th running of the Bank of America Chicago Marathon, the horrible events that took place in Las Vegas on Sunday are weighing heavily on our hearts," Executive Race Director Carey Pinkowski said in a statement. "We extend our deepest condolences to the victims, their families and all who have been affected by this national tragedy. We also understand how many who plan to participate in Sunday's celebration may have some concerns about public safety in the wake of Sunday's events." 

Pinkowski said organizers were discussing with authorities what adjustments would be made following the deadly concert shooting. 

"And, on race day, we will be working alongside the Chicago Police Department and many others in the law enforcement community, as we do every year, to keep our event, our participants and our city safe," he said. "We remind everyone on race day to report any suspicious activity to law enforcement immediately. And, we encourage all participants to join us on Sunday for what is always an uplifting, joyous celebration of the human spirit."

A planning exercise with both city officials and Chicago Marathon race organizers took place Tuesday.

"The planning exercise, with participation by both City officials and race organizers, is designed to analyze potential emergency events and existing emergency response plans to determine if changes or enhancements to the response plans are needed," the mayor's office said in a statement. "Chicago's emergency response plan has been recognized as a best practice by federal and international industry officials."

The Marathon won't be the only big event taking place in Chicago in the days following the tragic event.

The Cubs will also kick off their first playoff appearance at Wrigley Field this postseason, drawing large crowds of fans looking to cheer on the beloved North Siders. The city also plans to host watch parties at the Park at Wrigley during road games this postseason. 

A Cubs spokesperson said earlier this week the team has been discussing postseason preparedness with the city for weeks. 

Emanuel added the city has run six active shooter drills in the past year involving police, fire, OEMC and area hospitals.

Senior law enforcement officials briefed on the investigation told NBC News Thursday the gunman who killed dozens at the country music festival in Las Vegas Sunday researched Chicago's Lollapalooza in the months before the massacre. 

The sources said Paddock researched the Chicago festival and hotels around Boston's Fenway Park.

The motive behind the shooting remained unclear and officials could not immediately comment on why Paddock looked into the two locations.

Chicago police said they were looking into reports that Paddock booked rooms at a hotel overlooking Grant Park, where Lollapalooza was held Aug. 3 through 6, drawing major celebrity acts and fans, including former first daughter Malia Obama.

A law enforcement source confirmed Paddock booked two adjoining rooms at the Blackstone Hotel from Aug. 1-3, but never showed up.

The hotel did not immediately respond to a request for comment on the matter.