As another death was blamed on Hurricane Harvey on Monday, Texas officials defended their decision not to order a mandatory evacuation for millions of Houston-area residents.

A 60-year-old woman in east Montgomery County, north of Houston, died when a tree fell on her while she slept, the sheriff's office said, raising the confirmed number of deaths to three as historic flooding and unrelenting rains continued to cripple the region.

Texas Gov. Greg Abbott said at a news conference Monday that he had activated the entire 12,000-member Texas National Guard to help respond to recovery efforts in the disaster, which is potentially affecting more than 50 counties.

The "biggest challenge, first, is making sure we get everyone to a safe place," Abbott told reporters before touring the coastal community of Rockport, where Harvey made landfall as a powerful Category 4 hurricane Friday.

Mandatory evacuations were ordered for several counties along Texas' Gulf coast just hours before the hurricane made landfall.

But further inland, in cities like Houston, which is in Harris County, evacuation orders were never given. This is because of the uncertainty of the storm and the narrow window given to move millions of people, Harris County Judge Ed Emmett said on NBC's TODAY.

Emmett, the chief executive officer of Harris County, coordinates four county commissioners while overseeing local government actions.

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"When a hurricane comes, you know to get people out of harm's way, you know where the storm surge is going to hit. So we have a very defined process for moving people out of storm surge," Emmett said.

"With a rain event over a county of 4½ million people, you don't know exactly where the rain is going to fall, you don't know which neighborhoods are going to flood, so if we had gone out three days before and said we want 4 million people to leave Harris County, that would have been a totally nonsensical thing to do."

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But that sentiment wasn't fully echoed by Abbott, who said at a news conference Friday, as the storm was bearing down, that residents needed to listen to their instincts, even if no official had given an order to evacuate.

A Texas National Guardsman carries a woman from her flooded home in Houston on Sunday following Hurricane Harvey. Lt. Zachary West / Army National Guard via Getty Images

"Even if an evacuation order hasn't been issued by your local official, if you're in an area between Corpus Christi and Houston, you need to strongly consider evacuating. What you don't know, and what nobody else knows right now, is the magnitude of flooding that will be coming," Abbott said Friday.

Abbott warned that if residents didn't leave, they could be putting themselves in harm's way. Voluntary evacuation orders have since been issued for small parts of the greater Houston area.

About 20 inches of rain fell on parts of Houston, causing widespread flooding, and more than 2,000 rescues have been performed. Helicopters landed on highways to get people out of the affected areas, while good Samaritans used kayaks and canoes to free people from the flooding.

But Houston Mayor Sylvester Turner stood by his decision not to order the evacuation, saying it was impossible to put millions of people on the road.

Turner said at a news conference on Sunday that he had no regrets about telling people to stay put.

"One, if you can recall, there was a lot of conversation about the direction in which Hurricane Harvey was going to go," Turner said. "No one knew which direction it was going to go. So it's kind of difficult to send people away from danger when you don't know where the danger is. Number two, that kind of evacuation in a couple days — the logistics would have been crazy."

Emmett agreed with Turner's decision, saying it would have been foolish to send people away when the storm's path was unclear.

"We didn't know where it was going to flood. We didn't know what neighborhoods were going to flood, and the truth is, if we had told people to do that, they probably would have either laughed at us or ignored it," Emmett said.

Related: Everything You Need to Know About Hurricane Harvey

Turner's decision not to issue a sweeping evacuation of Houston was partly based on the chaos caused by a mandatory evacuation that was ordered in 2005 as Category 5 Hurricane Rita barreled toward Texas.

"Chaotic 2005 traffic with Hurricane Rita lurking was tragic. No official has issued evac order for Houston now. Calm and care! #Harvey," Turner tweeted on Friday.

As 2.5 million Houstonians fled, gridlock lasting nearly 24 hours trapped residents on highways and roads. The evacuation killed nearly as many people as the storm itself, which caused an estimated $12 billion in damage, according to The Houston Chronicle. Rita killed 120 people, according to HurricaneScience.org, an educational resource developed by the University of Rhode Island Graduate School of Oceanography.