In taking personal responsibility for approving the plan in the aftermath of Thursday's attack, Chief David Brown said he was convinced that the gunman would have sought to harm other police officers if he had hesitated to give the go-ahead.
But a search of Johnson's Dallas-area home after he was ultimately killed by police turned up bomb-making materials and a manual in which he wrote about military tactics. Johnson scrawled the initials "RB", which authorities are working to decipher. Black men dying and being forgotten.
The Dallas community's "unity is reflective of who we are as Americans" during these trying times, said Obama, speaking Sunday in Madrid.
The Arlington Police Department has offered to work with the Secret Service to provide security during the president's visit, Brown said.
Brown said he became increasingly concerned that "at a split second, he would charge us and take out many more before we would kill him". Mr Brown said: "He seemed very in control and very determined about hurting more officers".
"For our officers, they were suspects", he said.
Brown authorized the use of a robot to detonate the bomb that killed Johnson, who - according to Brown - told the officers he had explosives of his own and was ready to kill more people. He said he was upset about the recent police shootings.
Police in the US city of Dallas, Texas, stepped up security Saturday after a new threat was received, two days after snipers killed five police officers during a mass protest in the city. "We had negotiated with him for about two hours, and he just basically lied to us-playing games, laughing at us, singing, asking how many [police officers] did he get and that he wanted to kill some more and that there were bombs there", Brown said.
Johnson, an army veteran who served in Afghanistan, took advantage of a spontaneous march that began toward the end of the protest over those killings.
Police have been tight-lipped about exactly what they're investigating and what they've uncovered so far.
"When all attempts to negotiate with the suspect, Micah Johnson, failed under the exchange of gunfire, the department utilized the mechanical tactical robot, as a last resort, to deliver an explosion device to save the lives of officers and citizens", the police statement said.
Brown said it remains "undetermined" if Johnson also meant to target civilians.
He also told CNN that during the roughly two-hour standoff in the garage, Johnson lied to and taunted the police negotiators. "His plan was to kill as many as he could".