DALLAS (CBSNewYork/AP) — Federal investigators are working with local law enforcement in Dallas to try to piece together what led up to a deadly attack that killed five police officers in Dallas Thursday night.

Investigators have revealed a possible motive and sources said the gunman is believed to have acted alone.

READ MORE: Woman Collapses, Dies While Climbing Stairs To 19th-Floor Apartment During Power Outage In Co-Op City

MORE: CBS DFW | CBSN | Photos: Attack On Police In Dallas

Dallas Mayor Mike Rawlings said gunman Micah Xavier Johnson, 25, of Mesquite, Texas, acted alone in the shooting rampage that killed five officers and wounded seven more, CBS News’ Jessica Layton reported.

The 12 officers were shot just a few blocks from where President John F. Kennedy was slain in 1963. Authorities have said the seven wounded are expected to survive.

The attack marked the deadliest day for U.S. law enforcement since the Sept. 11, 2001, terrorist attacks.

Police initially had believed there had been more than one sniper, but sources said the ricochet of bullets had made it sound like more shooters were involved, sources said.

Dallas Police Chief David Brown reported that Johnson, a former member of the Army Reserve, said he was upset over recent police shootings of black men and wanted to kill white people. At a news conference Friday, Brown said the suspect made the comments before he was killed during a standoff with police in a downtown Dallas parking garage.

“There are no words to describe the atrocity that occurred to our city,” Brown said.

A U.S. intelligence source further told CBS News that Johnson had been planning to do something for a while – though it is not clear exactly how long. Law enforcement will check where and when he bought his guns, vest, and bomb-making material to determine how long he has been planning an attack.

On Friday, investigators found an arsenal of weapons stockpiled in Johnson’s home, including bomb-making materials, rifles and a journal of combat tactics, CBS News’ Don Champion reported. He followed black militant groups on social media, including one that posted a message Wednesday encouraging violence against police. A law enforcement source told CBS News that Johnson had an SKS semi-automatic assault rifle and a handgun. He also wore body armor.

Johnson was a private first class with a specialty in carpentry and masonry. He served in the Army Reserve for six years starting in 2009 and did one tour in Afghanistan from November 2013 to July 2014, the military said.

READ MORE: Exclusive: Orange County COVID Vaccine Site Ready To Go, But Organizers Say State Won't Send Them Any Doses

Bradford Glendening, a military lawyer, said Johnson was accused of sexual harassment by a female solider in Afghanistan. Johnson was sent back to the U.S. with the recommendation he be removed from the Army with an “other than honorable” discharge, but he later got an honorable discharge, Glendening said.

Brown said his department used a robot-delivered bomb to kill the suspect after hours of negotiations failed.

“We saw no other option but to use our bomb robot and place a device on its extension for it to detonate where the suspect was,” Brown said. “Other options would’ve exposed our officers to grave danger.”

Police said that no explosives were found after extensive sweeps of downtown areas.

Tther attacks on police included a Georgia man who authorities said called 911 to report a break-in, then ambushed the officer who came to investigate. That sparked a shootout in which both the officer and suspect were wounded but expected to survive.

In suburban St. Louis, a motorist shot an officer at least once as the officer walked back to his car during a traffic stop, police said. The officer was hospitalized in critical condition.

And in Tennessee, a man accused of shooting indiscriminately at passing cars and police on a highway told investigators he was angry about police violence against African-Americans, authorities said.

Protesters in cities across the nation continued to march against police brutality last night, with demonstrations in Atlanta, Phoenix, Minnesota, Washington D.C. and New York City.

“America is weeping,” said Rep. G.K. Butterfield, head of the Congressional Black Caucus, reflecting mounting despair.

President Barack Obama and Texas Gov. Greg Abbott asked for the public’s prayers. In a letter posted online Friday, Abbott said “every life matters” and urged Texans to come together.

“In the end,” he wrote, “evil always fails.”

MORE NEWS: New York City Schools Chancellor Richard Carranza Stepping Down

(TM and © Copyright 2016 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2016 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)